Replace old audio subsystem with SDL2 audio subsystem from Yamagi Quake II

-Fixes a bunch of issues with the existing audio subsystem
-Significantly extends ogg playback support
-Made ogg vorbis support mandatory
-Retained the music/track##.ogg file scheme
-Retained existing s_sdldriver behavior (accepts 'default'/'auto')
-Fixed buffer not being cleared during the SDL2 audio callback
-Haven't verified the OpenAL support (but should work?)
Update OVR latency tester code to print results in center of screen

-Useless now, but will be handy with DK2