Update to SDL3

[flibitijibibo]

The ABI has officially frozen, so this should be safe to do - if we want to keep 2.0 support until the final launch it should be easy enough to do a BUILD_SDL3 option like I did for FNA3D and FAudio.

[NyakoFox]

Could this wait until all of my touch input PRs get merged? It'd probably be easier to update all in one go instead of having to update the current codebase, then all affected PRs.

[flibitijibibo]

Definitely; I probably won't start this until 3.2.1 at the earliest, so there's a lot of time to get everything else merged.