*now that you can grab "anywhere" in the region, trim edges are inferior because they are hard to hit
*modifier had to change anyway (since CTRL is used as a copy-drag on both mac and win), so that invalidates prior muscle memory
*potentially frees up modifier on the trim edge for some other purpose (anchored_trim)
Control Points should ignore x_constraint which is set in Drag::motion and potentially other places.
The only thing that should prevent CP sliding is can_slide which is false for the points at the ends of a region.
Ripple edit undo/redo needs a "recursive diff" of all regions
in the playlist. To work properly owned changes have to be
cleared first, previously unrelated edits were be collected too.
This includes a more consistent version of
7a89d56009 using rdiff() instead of explicitly
saving region diffs.
Previously x-axis threshold was in samples. This is
useless unless zoomed in to the max. So in most cases
the first-move was always in x-direction, making constrained
y-axis drags near impossible.
Furthermore the threshold for copy-drags was increased
as per #8686
Region positions were updated in the GUI, before the playlist
was catching up.
The butler thread reads a region using the region's new position,
but the playlist's old range.
Thread 1 (GUI)
```
#22 ARDOUR::Playlist::notify_layering_changed()
#26 ARDOUR::AudioPlaylist::region_changed
#27 ARDOUR::Playlist::region_changed_proxy
#35 ARDOUR::Region::send_change
#36 ARDOUR::Region::set_position
#37 RegionRippleDrag::remove_unselected_from_views
#38 RegionRippleDrag::finished
```
LayeringChanged() also triggers DiskIOProcessor::playlist_modified
which schedules a pending-override and summons the butler.
Note that when moving only a few regions the butler starts after all
updates have been completed.
Butler thread:
```
#4 ARDOUR::AudioRegion::read_at
#5 ARDOUR::AudioPlaylist::read
#6 ARDOUR::DiskReader::audio_read
#7 ARDOUR::DiskReader::overwrite_existing_audio
#8 ARDOUR::DiskReader::overwrite_existing_buffers
#9 ARDOUR::Track::overwrite_existing_buffers
#10 ARDOUR::Session::non_realtime_overwrite
```
Region read fails:
```
libs/ardour/audioregion.cc:503 assert (position >= _position);
(gdb) p position
$1 = 1312000
(gdb) p _position
$2 = {<PBD::PropertyTemplate<long>> = {_have_old = true,
_current = 1336000, _old = 1312000} }
```
The DraggingView's time_axis_view (index) is set to the size of _time_axis_views
in ::motion() near line 1149. This makes it invalid as index into the vector.
Playlist changes do not include region-property changes.
When inserting a region, position(s) of other Regions may
change and those changes have to be recorded explicitly (for now).
see also RegionRippleDrag.
This is in preparation to subscribe to playhead cursor position
changes in the recorder-UI.
This change also clean up the API, replacing a public variable
with a const access method and follows #12 of
https://ardour.org/styleguide.html
