* Move audio callback to the main thread (fixes the longstanding hang
issues when using OpenGL)
* 64ms on desktop isn't enough due to excess buffering going on in this
emulator. Set default to 128ms instead.
* Different audio approach is necessary due to the following: it sets a
fixed framerate of 60, then sends either too many or too few audio
samples per frame in order to force the frontend to run at the correct
speed (.e.g. making the frontend run in slow motion for games that run
at 30fps) - this is undesirable for libretro, we want a consistent
amount of audio samples per batch. See further comments in libretro.cpp
for more information
