actor: Provide a better get_paint_volume() implementation
actor: Provide a better get_paint_volume() implementation

ClutterActor can do better by default than just giving up immediately.

An actor can check for the clip region, and for its children's paint
volume, for instance.

Just these two should give us a better default implementation for newly
written code.

GNOME/clutter,djdeath/clutter,jigpu/clutter,heysion/clutter-clone,jigpu/clutter,ebassi/clutter,heysion/clutter-clone,kerrickstaley/clutter-vala,Distrotech/clutter,kerrickstaley/clutter-vala,jigpu/clutter,heysion/clutter-clone,kerrickstaley/clutter-vala,ebassi/clutter,heysion/clutter-clone,djdeath/clutter,djdeath/clutter-android,djdeath/clutter-android,jigpu/clutter,GNOME/clutter,djdeath/clutter-multithreaded,djdeath/clutter-android,collects/clutter,collects/clutter,ebassi/clutter,collects/clutter,GNOME/clutter,djdeath/clutter,ebassi/clutter,djdeath/clutter,GNOME/clutter,djdeath/clutter,Distrotech/clutter,djdeath/clutter-android,jigpu/clutter,djdeath/clutter-android,collects/clutter,djdeath/clutter-multithreaded,ebassi/clutter,collects/clutter,Distrotech/clutter,GNOME/clutter,jigpu/clutter,djdeath/clutter-multithreaded,Distrotech/clutter,djdeath/clutter,djdeath/clutter-multithreaded,Distrotech/clutter,Distrotech/clutter,kerrickstaley/clutter-vala,heysion/clutter-clone