stage: Report an unknown paint volume.
stage: Report an unknown paint volume.

Previously we were leaving it up to the default implementation of
get_paint_volume in ClutterGroup to handle the stage by determining the
bounding box of all contained children. This isn't the true bounding box
of the stage though since the stage is responsible for clearing the
entire framebuffer at the start of the frame. This adds a
get_paint_volume implementation for ClutterStage which simply returns
False which means Clutter has to assume it covers everything.

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