`ExtractedView`s, not `ViewTarget`s.

OK, so this is tricky. Every frame, `delete_old_work_item_buffers`
deletes the mesh preprocessing index buffers (a.k.a. work item buffers)
for views that don't have `ViewTarget`s. This was always wrong for
shadow map views, as shadow maps only have `ExtractedView` components,
not `ViewTarget`s. However, before bevyengine#16836, the problem was masked,
because uploading the mesh preprocessing index buffers for shadow views
had already completed by the time `delete_old_work_item_buffers` ran.
But PR bevyengine#16836 moved `delete_old_work_item_buffers` from the
`ManageViews` phase to `PrepareResources`, which runs before
`write_batched_instance_buffers` uploads the work item buffers to the
GPU.

This itself isn't wrong, but it exposed the bug, because now it's
possible for work item buffers to get deleted before they're uploaded in
`write_batched_instance_buffers`. This is actually intermittent! It's
possible for the old work item buffers to get deleted, and then
*recreated* in `batch_and_prepare_binned_render_phase`, which runs
during `PrepareResources` as well, and under that system ordering, there
will be no problem other than a little inefficiency arising from
recreating the buffers every frame. But, if
`delete_old_work_item_buffers` runs *after*
`batch_and_prepare_render_phase`, then the work item buffers
corresponding to shadow views will get deleted, and then the shadows
will disappear.

The fact that this is racy is what made it look like bevyengine#16922 solved the
issue. In fact, it didn't solve the issue: it just perturbed the
ordering on the build bots enough that the issue stopped appearing.
However, on my system, the shadows still don't appear with bevyengine#16922.

This commit solves the problem by making `delete_old_work_item_buffers`
look at `ExtractedView`s, not `ViewTarget`s, preventing work item
buffers corresponding to live shadow map views from being deleted.

…actedView`s, not `ViewTarget`s. (#17039)

OK, so this is tricky. Every frame, `delete_old_work_item_buffers`
deletes the mesh preprocessing index buffers (a.k.a. work item buffers)
for views that don't have `ViewTarget`s. This was always wrong for
shadow map views, as shadow maps only have `ExtractedView` components,
not `ViewTarget`s. However, before #16836, the problem was masked,
because uploading the mesh preprocessing index buffers for shadow views
had already completed by the time `delete_old_work_item_buffers` ran.
But PR #16836 moved `delete_old_work_item_buffers` from the
`ManageViews` phase to `PrepareResources`, which runs before
`write_batched_instance_buffers` uploads the work item buffers to the
GPU.

This itself isn't wrong, but it exposed the bug, because now it's
possible for work item buffers to get deleted before they're uploaded in
`write_batched_instance_buffers`. This is actually intermittent! It's
possible for the old work item buffers to get deleted, and then
*recreated* in `batch_and_prepare_binned_render_phase`, which runs
during `PrepareResources` as well, and under that system ordering, there
will be no problem other than a little inefficiency arising from
recreating the buffers every frame. But, if
`delete_old_work_item_buffers` runs *after*
`batch_and_prepare_render_phase`, then the work item buffers
corresponding to shadow views will get deleted, and then the shadows
will disappear.

The fact that this is racy is what made it look like #16922 solved the
issue. In fact, it didn't: it just perturbed the ordering on the build
bots enough that the issue stopped appearing. However, on my system, the
shadows still don't appear with #16922.

This commit solves the problem by making `delete_old_work_item_buffers`
look at `ExtractedView`s, not `ViewTarget`s, preventing work item
buffers corresponding to live shadow map views from being deleted.

Currently, our batchable binned items are stored in a hash table that
maps bin key, which includes the batch set key, to a list of entities.
Multidraw is handled by sorting the bin keys and accumulating adjacent
bins that can be multidrawn together (i.e. have the same batch set key)
into multidraw commands during `batch_and_prepare_binned_render_phase`.

This is reasonably efficient right now, but it will complicate future
work to retain indirect draw parameters from frame to frame. Consider
what must happen when we have retained indirect draw parameters and the
application adds a bin (i.e. a new mesh) that shares a batch set key
with some pre-existing meshes. (That is, the new mesh can be multidrawn
with the pre-existing meshes.) To be maximally efficient, our goal in
that scenario will be to update *only* the indirect draw parameters for
the batch set (i.e. multidraw command) containing the mesh that was
added, while leaving the others alone. That means that we have to
quickly locate all the bins that belong to the batch set being modified.

In the existing code, we would have to sort the list of bin keys so that
bins that can be multidrawn together become adjacent to one another in
the list. Then we would have to do a binary search through the sorted
list to find the location of the bin that was just added. Next, we would
have to widen our search to adjacent indexes that contain the same batch
set, doing expensive comparisons against the batch set key every time.
Finally, we would reallocate the indirect draw parameters and update the
stored pointers to the indirect draw parameters that the bins store.

By contrast, it'd be dramatically simpler if we simply changed the way
bins are stored to first map from batch set key (i.e. multidraw command)
to the bins (i.e. meshes) within that batch set key, and then from each
individual bin to the mesh instances. That way, the scenario above in
which we add a new mesh will be simpler to handle. First, we will look
up the batch set key corresponding to that mesh in the outer map to find
an inner map corresponding to the single multidraw command that will
draw that batch set. We will know how many meshes the multidraw command
is going to draw by the size of that inner map. Then we simply need to
reallocate the indirect draw parameters and update the pointers to those
parameters within the bins as necessary. There will be no need to do any
binary search or expensive batch set key comparison: only a single hash
lookup and an iteration over the inner map to update the pointers.

This patch implements the above technique. Because we don't have
retained bins yet, this PR provides no performance benefits. However, it
opens the door to maximally efficient updates when only a small number
of meshes change from frame to frame.

The main churn that this patch causes is that the *batch set key* (which
uniquely specifies a multidraw command) and *bin key* (which uniquely
specifies a mesh *within* that multidraw command) are now separate,
instead of the batch set key being embedded *within* the bin key.

In order to isolate potential regressions, I think that at least #16890,
#16836, and #16825 should land before this PR does.

## Migration Guide

* The *batch set key* is now separate from the *bin key* in
`BinnedPhaseItem`. The batch set key is used to collect multidrawable
meshes together. If you aren't using the multidraw feature, you can
safely set the batch set key to `()`.

A previous PR, bevyengine#14599, attempted to enable lightmaps in deferred mode,
but it still used the `OpaqueNoLightmap3dBinKey`, which meant that it
would be broken if multiple lightmaps were used. This commit fixes that
issue, and allows bindless lightmaps to work with deferred rendering as
well.

…actedView`s, not `ViewTarget`s. (bevyengine#17039)

OK, so this is tricky. Every frame, `delete_old_work_item_buffers`
deletes the mesh preprocessing index buffers (a.k.a. work item buffers)
for views that don't have `ViewTarget`s. This was always wrong for
shadow map views, as shadow maps only have `ExtractedView` components,
not `ViewTarget`s. However, before bevyengine#16836, the problem was masked,
because uploading the mesh preprocessing index buffers for shadow views
had already completed by the time `delete_old_work_item_buffers` ran.
But PR bevyengine#16836 moved `delete_old_work_item_buffers` from the
`ManageViews` phase to `PrepareResources`, which runs before
`write_batched_instance_buffers` uploads the work item buffers to the
GPU.

This itself isn't wrong, but it exposed the bug, because now it's
possible for work item buffers to get deleted before they're uploaded in
`write_batched_instance_buffers`. This is actually intermittent! It's
possible for the old work item buffers to get deleted, and then
*recreated* in `batch_and_prepare_binned_render_phase`, which runs
during `PrepareResources` as well, and under that system ordering, there
will be no problem other than a little inefficiency arising from
recreating the buffers every frame. But, if
`delete_old_work_item_buffers` runs *after*
`batch_and_prepare_render_phase`, then the work item buffers
corresponding to shadow views will get deleted, and then the shadows
will disappear.

The fact that this is racy is what made it look like bevyengine#16922 solved the
issue. In fact, it didn't: it just perturbed the ordering on the build
bots enough that the issue stopped appearing. However, on my system, the
shadows still don't appear with bevyengine#16922.

This commit solves the problem by making `delete_old_work_item_buffers`
look at `ExtractedView`s, not `ViewTarget`s, preventing work item
buffers corresponding to live shadow map views from being deleted.