When we are painting a stack of 5-10 maximized windows, the
standard bottom-to-top method of drawing every actor results
in a tremendous amount of overdraw and can easily max out
the available memory bandwidth on a low-end* graphics chipset.
It's even worse if window textures are being accessed over
the AGP bus.

When we have opaque windows, we can go ahead and compute visibility
ourselves (in classic X-server fashion) and use that information to
restrict drawing obscured actors.

* Add MutterWindowGroup - a ClutterGroup subclass with logic
  for figuring out obscured regions.

* Add mutter_window_get_obscured_region() to get the region
  obscured by that window.

* Add mutter_shaped_texture_set_clip_region() to hint
  a clip region to the painting code; this is set based on
  the computed visible region of MutterWindowGroup.

* Add tidy_texture_frame_set_needs_paint() to hint that the
  paint can be skipped entirely; this is used when we detect
  that the window shadow is entirely obscured.

http://bugzilla.gnome.org/show_bug.cgi?id=587344