Lots of applications often use GtkFrame as a giant toplevel container,
and that means that they get size allocated often. When frames get size
allocated, they invalidate their entire widget tree, even if they
haven't changed size or anything like that happens at all. This is
because the shadow / label needs to be redrawn if the child changes
size. We can optimize this out and only mark ourselves for a redraw if
the child has actually changed its size.