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.
This was changed in fc9840f2a9 to hard code a "false" argument describing
whether or not the child bbox had changed. This causes the parent to not
recompute its own bounding box when a child is shown/hidden, which is
incorrect. We now force it to true, which may be overkill for some purposes,
but at worst will just force an unnecessary bbox re-computation and nothing
more.
the dumb/Item version of ::size_allocate_children() doesn't do anything but give its full allocation to children, and if there
was more than one child, this could not be done in a "dumb" way. Likewise, if the sole child is not layout sensitive, it
doesn't care about size allocation
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
Called when an item has requested a redraw and intersects with visible
canvas area.
Also add Canvas::prepare_for_render that will call Item::prepare_for_render for
items visible on the canvas.
