Track instruments in sequencer playback, and if a music stream has been changed so that instruments differ, retrigger relevant notes.  Fix assertions when updating playback state.  Updating music streams on the fly is not correct yet, but at least more so.
Track instruments in sequencer playback, and if a music stream
has been changed so that instruments differ, retrigger relevant
notes.  Fix assertions when updating playback state.  Updating
music streams on the fly is not correct yet, but at least more so.

turnipsi/mdl,turnipsi/mdl