This moves MIDI channel filtering into a reusable class and moves filtering to
the source, rather than modifying the buffer afterwards. This is necessary so
that the playlist trackers reflect the emitted notes (and thus are able to stop
them in situations like mute).
As a perk, this is also faster because events are just dropped on read, rather
than pushed into a buffer then later removed (which is very slow).
Really hammering on mute or solo still seems to produce stuck notes
occasionally (perhaps related to multiple-on warnings). I am not yet sure why,
but occasional beats always.
During DnD, the region uses the 'old/current'
midi_stream_view()'s range and its position/height calculation.
Ideally DnD would decouple the midi_stream_view() for the
region(s) being dragged and set it to the target's range
(or in case of the drop-zone, FullRange).
but I don't see how this can be done without major rework.
For now, just prevent visual bleeding of events in case
the target-track is smaller.
* Handle large (delta > 1) movements into the DZ
which are not due to invalid-drop positions, but
caused by laggy GUI or rapid user movements.
* ignore busses when moving out of the DZ.
Allow to drag multiple regions from different tracks
to/from the dropzone.
Busses & Automation-lanes are ignored, as are
hidden tracks.
Any region may serve as mouse drag anchor.
fixes#6172 and #6176
Fixes bug #6214.
It would be better to do this while dragging, but this would require rewriting
much of the drag code to keep track of a cumulative y delta since the current
position of points would be "sticky" and prevent any movement at all, so this
will have to do for now.
