Swift library for SDL2 and SDL3

Each capability is its own product, so clients only pull in the dependencies (and system libraries) they actually need:

• SDL2Swift / SDL3Swift: Swift wrapper for SDL2 / SDL3 core.
• SDL2Image / SDL3Image: Swift wrapper for image loading via SDL_image, depends on SDL2Swift / SDL3Swift.
• SDL2Mixer: Swift wrapper for SDL_mixer's classic Mix_* audio API, depends on SDL2Swift.
• SDL3Mixer: Swift wrapper for SDL_mixer's new MIX_* audio API, depends on SDL3Swift.
• CSDL2Image / CSDL3Image: raw C module for SDL_image, for direct interop.
• CSDL2Mixer / CSDL3Mixer: raw C module for SDL_mixer, for direct interop.