This adds a new dependency with libfuildsynth. MIDI support
is built by default, but can be disabled if not desired.
All RTP songs should work well, but there are known problems
with other files (see README). Also, the pitch shift implementation
is somewhat poor and doesn't match RMXP (at least subjectively).
A soundfont is not included and must be provided by
the user themself.
Very rarely rogue buffers would remain and play on loop on song
switch because we only ever cleared processed, not queued, buffers
from the source.
The correct way to completely clear a source's queue is to
simply attach a null buffer to it.
