debug: disable culling with CLUTTER_PAINT=paint-volumes

When using CLUTTER_PAINT=paint-volumes to visualize the paint-volumes of
actors we were already disabling clipped-redraws because we are drawing
extra geometry that the actors don't know about but we didn't disable
culling. This was resulting in actors disappearing while using this
debug option.

clutter/clutter-main.c

@@ -1236,7 +1236,10 @@ clutter_init_real (GError **error)
 
   /* The same is true when drawing the outlines of paint volumes... */
   if (clutter_paint_debug_flags & CLUTTER_DEBUG_PAINT_VOLUMES)
-    clutter_paint_debug_flags |= CLUTTER_DEBUG_DISABLE_CLIPPED_REDRAWS;
+    {
+      clutter_paint_debug_flags |=
+        CLUTTER_DEBUG_DISABLE_CLIPPED_REDRAWS | CLUTTER_DEBUG_DISABLE_CULLING;
+    }
 
   /* this will take care of initializing Cogl's state and
    * query the GL machinery for features