Change the way the update area is tracked during paint

First of all we track the current update area during an
update in window->active_update_area. This will be used later
in end_paint to know the damaged area.

Secondly we keep track of old update areas for the last 2
frames. This will later allow us to reuse old framebuffer
contents in double or tripple buffer setups, only painting
what has changed since then.

gdk/gdkinternals.h

@@ -213,6 +213,12 @@ struct _GdkWindow
 
   cairo_region_t *update_area;
   guint update_freeze_count;
+  /* This is the update_area that was in effect when the current expose
+     started. It may be smaller than the expose area if we'e painting
+     more than we have to, but it represents the "true" damage. */
+  cairo_region_t *active_update_area;
+  /* We store the old expose areas to support buffer-age optimizations */
+  cairo_region_t *old_updated_area[2];
 
   GdkWindowState state;