audio: Redesigned audio conversion code for SDL3.

- SDL_AudioCVT is gone, even internally.
- libsamplerate is gone (I suspect our resampler is finally Good Enough).
- Cleanups and improvements to audio conversion interfaces.
- SDL_AudioStream can change its input/output format/rate/channels on the fly!

src/audio/SDL_mixer.c

@@ -81,6 +81,9 @@ static const Uint8 mix8[] = {
 #define ADJUST_VOLUME(type, s, v) ((s) = (type)(((s) * (v)) / SDL_MIX_MAXVOLUME))
 #define ADJUST_VOLUME_U8(s, v)    ((s) = (Uint8)(((((s) - 128) * (v)) / SDL_MIX_MAXVOLUME) + 128))
 
+
+/* !!! FIXME: this needs some SIMD magic. */
+
 int SDL_MixAudioFormat(Uint8 *dst, const Uint8 *src, SDL_AudioFormat format,
                         Uint32 len, int volume)
 {