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!

CMakeLists.txt

@@ -411,8 +411,6 @@ set_option(SDL_PULSEAUDIO          "Use PulseAudio" ${UNIX_SYS})
 dep_option(SDL_PULSEAUDIO_SHARED   "Dynamically load PulseAudio support" ON "SDL_PULSEAUDIO" OFF)
 set_option(SDL_SNDIO               "Support the sndio audio API" ${UNIX_SYS})
 dep_option(SDL_SNDIO_SHARED        "Dynamically load the sndio audio API" ON "SDL_SNDIO" OFF)
-set_option(SDL_LIBSAMPLERATE       "Use libsamplerate for audio rate conversion" ${UNIX_SYS})
-dep_option(SDL_LIBSAMPLERATE_SHARED "Dynamically load libsamplerate" ON "SDL_LIBSAMPLERATE" OFF)
 set_option(SDL_RPATH               "Use an rpath when linking SDL" ${UNIX_SYS})
 set_option(SDL_CLOCK_GETTIME       "Use clock_gettime() instead of gettimeofday()" ${SDL_CLOCK_GETTIME_ENABLED_BY_DEFAULT})
 set_option(SDL_X11                 "Use X11 video driver" ${UNIX_SYS})
@@ -2909,7 +2907,6 @@ if(HAVE_VULKAN AND NOT SDL_LOADSO)
 endif()
 
 # Platform-independent options
-CheckLibSampleRate()
 
 if(SDL_VIDEO)
   if(SDL_OFFSCREEN AND SDL_VIDEO_OPENGL_EGL)