window-actor: Fix optimization in get_paint_volume

We need to clip the paint volume to the unobscured region, not the
other way around...

https://bugzilla.gnome.org/show_bug.cgi?id=720630
window-actor: Fix unobscured_region handling when computing  paint volume

We currently ignore the unobscured region when we have mapped clones in
meta_window_actor_process_damage and meta_window_actor_damage_all but
use it unconditionally when computing the paint volume.

This is wrong. We should ignore it there as well or we will end up with
empty clones if the cloned window is completly obscured
(like the tray icons in gnome-shell).

https://bugzilla.gnome.org/show_bug.cgi?id=721596