primitives: avoid leaking polygon override materials
primitives: avoid leaking polygon override materials

If we have to make override changes to the user's source material to
handle cogl_polygon then we need to make sure we unref the override
material at the end.

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