Reconsider how we handle audio callbacks

[icculus]

3. We could have sdl2-compat manage the SDL2 callback itself and feed the audio stream at a consistent rate, and not use the audio stream callback at all.

4. We could wait to fix this until we add a postmix callback to each logical device in SDL3, which would probably map much more closely to SDL2's audio callback, and make sdl2-compat use that instead. If this gets added, it's going to be unforgiving about data conversion and such, because we just built a whole fancy layer on top of it to handle that. But that would be sdl2-compat's problem, not the SDL2-based app's.

Originally posted by @icculus in https://github.com/libsdl-org/sdl2-compat/issues/94#issuecomment-1670448845

[icculus]

(These changes might not be necessary or desirable, in which case I'll close this later, but I don't want to lose these ideas until I've thought about them more.)