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