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.
Child items will be hidden when their ancestors are hidden. The old ::visible() implementation didn't reflect this. In addition,
when changes are made to hidden items (new definition of visible/not visible), don't bother to request redraws, since this will
be done when the item becomes visible again.
Remove Canvas::Layout, use Canvas::Container for the same purpose, move child-rendering into Item::render_children() so that it
could theoretically be used by any derived type.
Items no longer need a parent group (they require a Canvas pointer instead), so all constructors have been rationalized
and have two variants, one with a parent and one with a canvas.
All Items now inherit from Fill and Outline, to banish diagonal inheritance and virtual base classes and all that.
There were zero changes to the Ardour GUI arising from these changes.
The idea now is that a scroll group item can be added to the canvas which will causes its children to scroll in either or both
directions (horizontal or vertical). There are few complications: the position() of the ScrollGroup is ambiguous depending
on whether you want it with scroll taken into account or not, so Item::canvas_position() was added, which defaults to
the same value as Item::position() but is overridden by ScrollGroup to return the position independent of scrolling. This
method is used when translating between item/canvas/window coordinate systems.
Note that the basic idea is that we MOVE the scroll group when a scroll happens. This mirrors what happens in the GnomeCanvas,
where Nick Mainsbridge came up with a great idea that allowed unification of the time bar and track canvases.
