[animation] Add Animation getter

If we are animating an actor using the clutter_actor_animate*() family
of functions we might want to expose a getter for the Animation instance
we are using, to avoid excessing bookkeeping.

clutter/clutter-animation.h

@@ -163,6 +163,8 @@ ClutterAnimation *   clutter_actor_animate_with_alphav    (ClutterActor
                                                            const gchar * const   properties[],
                                                            const GValue         *values);
 
+ClutterAnimation *   clutter_actor_get_animation          (ClutterActor         *actor);
+
 G_END_DECLS
 
 #endif /* __CLUTTER_ANIMATION_H__ */