We need a MidiStateTracker to determine notes whose end is not reached
during the call to ::write_one_track(), so that we can resolve them
in the output (SMF) source. This required some changes to the ::export_stuff()
API for tracks.
In addition, we now take the source "lock" just once during ::write_one_track()
rather than every time we write. This isn't an integral part of the
note tracking, but fell out along the way.
Finally, note that although we use a vector to handle MIDI "sources" here,
it is expected that there is only 1 MIDI source at present. Leave vectors in
place since it is possible that ::write_one_track() could be modified
in the future to change that.
9b070eefb1