Implement Web Audio API playback backend to SDL Mix_* interface. This fixes https://github.com/kripken/emscripten/commit/5b41371bb5897a49e1fdcad18d053f775105c846 when Web Audio API is available, and works around the Firefox bug https://bugzilla.mozilla.org/show_bug.cgi?id=654787.
Merge pull request #2361 from juj/sdl_mix_webaudio

Sdl mix webaudio