cogl: Create CoglTextureUnit with its associated unit number
cogl: Create CoglTextureUnit with its associated unit number

The index field of CoglTextureUnit was never set, leading to the
creation of units with index set to 0. When trying to retrieve a texture
unit by its index (!= 0) with _cogl_get_texture_unit(), a new one was
created as it could not find it back in the list of textures units:
ctx->texture_units.

http://bugzilla.openedhand.com/show_bug.cgi?id=1958

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