stop using vertical adjustment to account for scrolling, since event coordinates for all events that are delivered to the canvas are adjusted before Editor sees them
manual translate events in Editor::track_canvas_scroll() to canvas space, because the event arrives there directly, not via the canvas. no other similar eventhandler in that code uses the event coordinates
remove TimeAxisView::clip_to_viewport() and Editor::update_canvas_now() and Editor::flush_canvas() which should no longer be necessary with a sane canvas design