audio/sdl3_mixer: New port, add sdl3_mixer 3.2.4 to tree

Audio management library providing decoding of many popular audio file
formats, mixing, various DSP processing effects and positional audio.
Audio data can be preloaded, or streamed on-the-fly into the mixer.
SDL3_mixer supports multiple audio devices, and can mix to a memory
buffer instead of feeding audio hardware in real time.