render: remove logical presentation render target

[icculus]

This is still totally busted even for simple cases, but here's testspriteminimal mostly working:

This isn't ready to merge, but it's been radio silence for way too long, so I'm putting it in a draft PR.

I spent time trying a few different approaches:

• Hook into the draw command queueing and adjust coords: this was a non-starter, since we'd have to diddle const data on its way through the system.
• Hook into just the setviewport queueing: the rest of the existing system fights this.
• Hook into each backend: lots of duplicated code and chances to get things wrong, plus all the problems from above.

The current approach tries to use a seperate SDL_RenderViewState to manage logical presentation, and this seems to work without a lot of code changes, but some math is wrong somewhere, still. There's a lot of state management that needs to be examined at the edges anyhow, like when a resize event comes through and possibly wipes out some view state, etc.

Also, if the app sets a viewport or scale of its own, it's going to cause problems, but eventually that would be managed as separate value that we just multiply in to the final calculation, but I need to get the math right on the normal codepath before I worry about that.

(eventually) Fixes #8736.

[slouken]

grin Now you know why I implemented it using a render target.

[icculus]

Alright, now we're getting somewhere:

More to do, but I think this was the hard part...?

[slouken]

Alright, now we're getting somewhere
More to do, but I think this was the hard part...?

Yay!