cogl-clip-stack: Set *stencil_used_p when the stack is empty
cogl-clip-stack: Set *stencil_used_p when the stack is empty

If the clip stack is empty then _cogl_clip_stack_flush exits
immediately. This was missing out the assignment of *stencil_used_p at
the bottom of the function. If a path is then used after the clip is
cleared then it would think it needs to merge with the clip so the
stencil would not be cleared correctly.

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