Persist shadow atlas allocations between frames

[tomas7770]

Problem

Currently, the shadow atlas plugin re-allocates space for every shadow map every frame. This is fine for small amounts of shadow casters, but can incur a performance cost as the amount grows.

Possible solution

Each shadow caster already stores an id value that is used to find its shadow map's location in the atlas. When the id is -1, it means that the shadow map hasn't been allocated yet. This can be used to detect new shadow casters being created between frames, and only allocate room for these.

Likewise, each possible location (Slot) for a shadow map contains the id of its corresponding caster. If no caster has this id, then the one that had it no longer exists.