Add PortMIDI music stubs.
Add native MIDI music driver stubs to the audio library.
Register native MIDI music driver.
Update the makefile.
Wrap SDL synchronization primitives.
Implement PortMidi music output.
Implement most native MIDI output functions in the sound library.
Fades are currently unsupported.
Prevent busy waiting during the Apogee intro fanfare.
Merge in preliminary MIDI support from audio.
Implement music fades for native MIDI.
Make native MIDI music the default.
Merge in remaining MIDI changes from audio.