osx: Fixed bug with creating context twice.
osx: Fixed bug with creating context twice.

We should assign context and pixel_format vars to null to avoid
multiple context creation.

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