actor: Allow querying the paint volume

An actor has an implicit "paint volume", that is the volume in 3D space
occupied when painting itself.

The paint volume is defined as a cuboid with the origin placed at the
top-left corner of the actor; the size of the cuboid is given by three
vectors: width, height and depth.

ClutterActor provides API to convert the paint volume into a 2D box in
screen coordinates, to compute the on-screen area that an actor will
occupy when painted.

Actors can override the default implementation of the get_paint_volume()
virtual function to provide a different volume.

clutter/clutter-private.h

@@ -409,6 +409,10 @@ gpointer _clutter_event_get_platform_data (const ClutterEvent *event);
 
 #endif
 
+ClutterPaintVolume *_clutter_paint_volume_new     (ClutterActor       *actor);
+void                _clutter_paint_volume_get_box (ClutterPaintVolume *pv,
+                                                   ClutterActorBox    *box);
+
 G_END_DECLS
 
 #endif /* _HAVE_CLUTTER_PRIVATE_H */