[actor] Allow querying whether we are painted by a Clone

It would be useful inside a custom actor's paint function to be able to
tell if this is a primary paint call, or if we are in fact painting on
behalf of a clone.

In Mutter we have an optimization not to paint occluded windows; this is
desirable for the windows per se, to conserve bandwith to the card, but
if something like an application switcher is using clones of these windows,
they will not get painted either; currently we have no way of
differentiating between the two.

Fixes bug:

  http://bugzilla.openedhand.com/show_bug.cgi?id=1685

doc/reference/clutter/clutter-sections.txt

@@ -387,6 +387,7 @@ clutter_actor_grab_key_focus
 clutter_actor_get_pango_context
 clutter_actor_create_pango_context
 clutter_actor_create_pango_layout
+clutter_actor_is_in_clone_paint
 
 <SUBSECTION>
 ClutterActorBox