our policy is that "mute works on outputs", which means that mute should have
no effect on what an instrument plugin does. However, MidiTrack::act_on_mute()
used to inject sustain=0 messages into the data flow, which would affect
the instrument plugin(s) in the track.
now, MidiTrack::act_on_mute() simply notifies all Delivery objects in the track
that a MIDI mute is needed via a channel mask. The Delivery objects notice this
during their ::run() method, and deliver the required MIDI events to their
output ports.
There is still a potential issue that Amp objects which notice they have been
muted also send a similar set of messages. This needs more investigation and
possibly other changes. But this commit allows a sustained note to return after
the track is muted midway through it.
8b09becf1d