window-props.c: React to changes to _GTK_FRAME_EXTENTS

When _GTK_FRAME_EXTENTS changes, we need to redo constraints on
the window - this matters in particular if the toolkit removes
invisible borders when a window is maximized, since otherwise
the maximized window will be positioned as if it still has
invisible borders.

https://bugzilla.gnome.org/show_bug.cgi?id=714707
MetaWindowGroup: fix paint volume

In the past, MetaWindowGroup was allocated the size of the screen and
painted the size of the screen because it contained the screen background,
but now we also have the "top window group" which contains only popup
windows, so the allocation doesn't properly reflect the paint bounds
of the window group. Compute the paint bounds accurately from the
children.

https://bugzilla.gnome.org/show_bug.cgi?id=719669