The paint volume structure is cached in the Actor it references, and
this causes a reference cycle.

The paint volume is going to be used when painting, so the actor must
still be valid - otherwise Clutter will bail out far before than
accessing the actor pointer in ClutterPaintVolume.

Otherwise, we could have used dispose() to check for a valid actor and
remove a reference if the actor field is !NULL; it feels less clean,
though, since we're effectively managing an extra reference on
ourselves.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2431