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!

docs/README-linux.md

@@ -16,7 +16,7 @@ Ubuntu 18.04, all available features enabled:
 
     sudo apt-get install build-essential git make \
     pkg-config cmake ninja-build gnome-desktop-testing libasound2-dev libpulse-dev \
-    libaudio-dev libjack-dev libsndio-dev libsamplerate0-dev libx11-dev libxext-dev \
+    libaudio-dev libjack-dev libsndio-dev libx11-dev libxext-dev \
     libxrandr-dev libxcursor-dev libxfixes-dev libxi-dev libxss-dev \
     libxkbcommon-dev libdrm-dev libgbm-dev libgl1-mesa-dev libgles2-mesa-dev \
     libegl1-mesa-dev libdbus-1-dev libibus-1.0-dev libudev-dev fcitx-libs-dev
@@ -32,15 +32,11 @@ Fedora 35, all available features enabled:
     systemd-devel mesa-libGL-devel libxkbcommon-devel mesa-libGLES-devel \
     mesa-libEGL-devel vulkan-devel wayland-devel wayland-protocols-devel \
     libdrm-devel mesa-libgbm-devel libusb-devel libdecor-devel \
-    libsamplerate-devel pipewire-jack-audio-connection-kit-devel \
+    pipewire-jack-audio-connection-kit-devel \
 
 NOTES:
 - The sndio audio target is unavailable on Fedora (but probably not what you
   should want to use anyhow).
-- libsamplerate0-dev lets SDL optionally link to libresamplerate at runtime
-  for higher-quality audio resampling. SDL will work without it if the library
-  is missing, so it's safe to build in support even if the end user doesn't
-  have this library installed.
 
 
 Joystick does not work

docs/README-migration.md

@@ -103,6 +103,9 @@ If you need to convert U16 audio data to a still-supported format at runtime, th
     }
 ```
 
+In SDL2, SDL_AudioStream would convert/resample audio data during input (via SDL_AudioStreamPut). In SDL3, it does this work when requesting audio (via SDL_GetAudioStreamData, which would have been SDL_AudioStreamPut in SDL2. The way you use an AudioStream is roughly the same, just be aware that the workload moved to a different phase.
+In SDL2, SDL_AudioStreamAvailable() returns 0 if passed a NULL stream. In SDL3, the equivalent SDL_GetAudioStreamAvailable() call returns -1 and sets an error string, which matches other audiostream APIs' behavior.
+
 
 The following functions have been renamed:
 * SDL_AudioStreamAvailable() => SDL_GetAudioStreamAvailable()