cogl-pipeline: Be careful not to take ownership of root layer
cogl-pipeline: Be careful not to take ownership of root layer

In _cogl_pipeline_prune_empty_layer_difference if the layer's parent
has no owner then it just takes ownership of it. However this could
theoretically end up taking ownership of the root layer because
according to the comment above in the same function that should never
have an owner. This patch just adds an extra check to ensure that the
unowned layer has a parent.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2588

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