Replace SDL_mixer with an audio implementation using libsndfile,
sts_mixer (a single-file header library) and SDL's basic audio
interface.
This allows for much more flexibility in controlling audio playback.
Fading is now properly implemented, as well as various other functions
from the SACT2/KiwiSoundEngine interface.
sts_mixer can support recursive mixing, so it will be possible to
implement the "VolumeValancer" mixer channels specified in
System40.ini in the future (once I figure out how the mapping of audio
files to mixer channels is supposed to work).
