If queue_draw is "frozen", we simply accumulate drawing
requests in a (union) rectangle, and when finally "thawed"
the canvas submits a single redraw request for the entire
accumulated rect.
Although in theory this is all that GTK/GDK does for
draw requests, callgrind reveals significant costs
associated with the actual calltree for GtkWidget::queue_draw_area().
One potential cost is that GDK also maintains a list of
invalidated rectangles in addition to the union, and
for MIDI regions with thousands of notes, this can represent
real overhead. This approach dispenses with the rect list,
since our Canvas drawing model only uses the union rectangle
anyway.
The AudioClipEditor features a scroll bar that is a part of the canvas. Because scroll
groups are at the top level of a canvas, the scroll bar is necessary within a scroll
group, which causes it to get confused about the difference between its own
position within the canvas and that of the scroll group. This commit introduces
a per-Item flag, _scroll_translation, which is true by default. If false, the
item will not have coordinates translated to reflect scroll group position.
We need to be able to construct Metrics after the ruler is constructed
in order to deal with Triggers which are potentially short-lived. We
stored the reference as a pointer anyway, so it's not as much of a change
as it appears.
If there's a grabbed item (GtkCanvas only at present) then unless it belongs to the scroll
group used for scroll offset translation, the event coordinates should not be translated,
even if the mouse pointer moves into the scroll group.
This previously wasn't done because of fear that it would affect the traditional fixed-sized canvas,
but only items that _layout_sensitive (i.e. are packed into a constraint packer directly) will
actually do anything in ::size_allocate().
Possibly might want to relax this to cover items that have a constraint packer between them
and a root group.
Items call ::queue_resize(), which sets a flag in the canvas; at next idle, we call
Canvas::layout() which walks the item tree and recursively calls layout (depth first)
on all items needing a resize.
Only Container types implement layout, and so far only Box
