[texture] Check if the material is valid
[texture] Check if the material is valid

The CoglMaterial handle might be invalid when we are disposing
the ClutterTexture, so we need to check before calling any
cogl_material_* API on it.

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