In _clutter_actor_set_default_paint_volume we were returning FALSE if an
actor has an empty allocation because we were claiming it doesn't have a
paint-volume. Actually an empty/degenerate pv is valid and has different
semantics to returning FALSE because FALSE means the pv is unknown and
so Clutter will have to assume the worst - that the pv is basically
un-bounded.

Reviewed-by: Emmanuele Bassi <ebassi@linux.intel.com>