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!

include/build_config/SDL_build_config.h.cmake

@@ -221,7 +221,6 @@
 
 #cmakedefine HAVE_LINUX_INPUT_H 1
 #cmakedefine HAVE_LIBUDEV_H 1
-#cmakedefine HAVE_LIBSAMPLERATE 1
 #cmakedefine HAVE_LIBDECOR_H  1
 
 #cmakedefine HAVE_D3D_H @HAVE_D3D_H@
@@ -543,9 +542,6 @@
 /* Whether SDL_DYNAMIC_API needs dlopen */
 #cmakedefine DYNAPI_NEEDS_DLOPEN  @DYNAPI_NEEDS_DLOPEN@
 
-/* Enable dynamic libsamplerate support */
-#cmakedefine SDL_LIBSAMPLERATE_DYNAMIC @SDL_LIBSAMPLERATE_DYNAMIC@
-
 /* Enable ime support */
 #cmakedefine SDL_USE_IME @SDL_USE_IME@