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
For Clone actors we will need a way to report the volume of the source
actor as the volume of the clone actor. To make this work though we need
to be able to replace the reference to the source actor with a reference
to the clone actor instead. This adds a private
_clutter_paint_volume_set_reference_actor function to do that.
This adds a way to initialize a paint volume from another source paint
volume. This lets us for instance pass the contents of one paint volume
back through the out param of a get_paint_volume implementation.
This splits out all the clutter_paint_volume code from clutter-actor.c
into clutter-paint-volume.c. Since clutter-actor.c and
clutter-paint-volume.c both needed the functionality of
_fully_transform_vertices, this function has now been moved to
clutter-utils.c as _clutter_util_fully_transform_vertices.
