actor: Fix get_paint_volume() default implementation

Providing a default get_paint_volume() that takes into account the
children of an actor was a goal of the whole First Apocalypse; if we
make all the containers rely on it, and yet we return a FALSE value
(meaning: we don't have a valid paint volume) even when we do have it,
then we are going to break the whole machinery, though.

clutter/clutter-actor.c

@@ -4403,6 +4403,7 @@ clutter_actor_real_get_paint_volume (ClutterActor       *self,
         }
 
       clutter_paint_volume_union (volume, child_volume);
+      res = TRUE;
     }
 
   return res;