add SDL_AudioQuit and SDL_VideoQuit

SDL_AudioQuit and SDL_VideoQuit were both declared but not defined in the SDL port. I added those and changed SDL_Quit to call SDL_AudioQuit.
this won't work for 2 reasons: first, as a C function it will have a `_` prefix. Second, a file may include SDL_Quit but not SDL_AudioQuit. (the current browser tests will catch this, `python tests/runner.py browser`)

You can fix the latter with `SDL_Quit__deps: ['SDL_AudioQuit']` so that it includes that function. Please run the tests to be sure it works.