window-actor: Use more conservative paint volume

We know which part of the window is not obscured, so take advantaged of that
to limit the size of the paint volume that we report.

src/compositor/meta-window-actor.c

@@ -734,6 +734,9 @@ meta_window_actor_get_paint_volume (ClutterActor       *actor,
       gdk_rectangle_union (&bounds, &shadow_bounds, &bounds);
     }
 
+  if (priv->unobscured_region)
+    cairo_region_intersect_rectangle (priv->unobscured_region, &bounds);
+
   origin.x = bounds.x;
   origin.y = bounds.y;
   origin.z = 0.0f;