remove irrelevant comment
fix up some logic that allowed hits (percussive note display) to be trimmed with the mouse, which was not intended
move an assert to cover only cases where it is needed
second part of the logic fix for percussive note trimming
third part of the logic fix for percussive note trimming
slightly optimize bounding box computation for ArdourCanvas::PolyItem by avoiding inner conditional
alter ArdourCanvas::Rect::expand() to not clamp to all-positive values because we now definitely have items that have negative coordinates.

This could have drastic consequences but I think the all-positive was a hack to fix a problem that was solved separately by means currently unknown.