Add SDL_mixer support

[PaulMDemers]

Summary

Adds SDL_mixer support to nxdk as an SDL2 companion library.

This first pass enables the core SDL_mixer library with WAV/chunk playback support only. Compressed/music codec support such as OGG, MP3, MOD, MIDI, and FLAC is intentionally deferred to follow-up work so the initial integration stays small and reviewable.

Changes

• Add lib/sdl/SDL_mixer submodule
• Build libSDL2_mixer.lib when NXDK_SDL = y
• Enable SDL_mixer core + MUSIC_WAV
• Add lib/pkgconfig/SDL2_mixer.pc
• Add samples/sdl_mixer
• Update README SDL library summary

SDL_mixer submodule

The SDL_mixer submodule points at PaulMDemers/SDL_mixer, branch nxdk-sdl-mixer.

That branch is based on SDL_mixer release-2.0.4 and contains one small nxdk compatibility change: use the existing strtok() path for XBOX builds instead of the Windows strtok_s() path.

Verification

Built successfully on Windows/MSYS2 with:

make -C samples/sdl_mixer

This produced:

• lib/libSDL2_mixer.lib
• samples/sdl_mixer/bin/default.xbe
• samples/sdl_mixer/nxdk sample - SDL2_mixer.iso

The sample generates a small in-memory WAV tone, loads it through Mix_LoadWAV_RW, and plays it through SDL_mixer.

[JayFoxRox]

I don't think this should be merged. See reasons in #601

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use the existing strtok() path for XBOX builds instead of the Windows strtok_s() path.

Sounds like upstream should check for __STDC_LIB_EXT1__ instead, then nxdk would work without changes.
nxdk could also add a dummy wrapper for strtok_s (upstream in pdclib?) or to the WIN32 code (as part of the win32 libc).

[tifasoftware]

Is there any recent developments to adding SDL2_mixer to the nxdk? My project requires SDL2_mixer, and I might possibly assist getting the full SDL2_mixer to the SDK.

[PaulMDemers]

@tifasoftware the work hasn't been forgotten, I am distracted with another project at the moment.

This port currently is just the initial base WAV support, additional help is more than welcome. :)

It'll be probably a week or two before I'm back working on libraries.

[tifasoftware]

That's absolutely okay, take your time. I too can get distracted with other parts of my game engine too. I'll probably be working on my other ports to the PS2, GameCube, and the Dreamcast in the meantime.

[tifasoftware]

My project also will need ogg support, but I'll try to help in any way i can in the future