paint-nodes: Have a fallback buffer for the root node
paint-nodes: Have a fallback buffer for the root node

If we don't get passed a CoglFramebuffer when creating the root paint
node then we ask Cogl to give us the current draw buffer.

This allows the text-cache conformance test to pass, but it'll require
further investigation.

ebassi/clutter,jigpu/clutter,jigpu/clutter,jigpu/clutter,djdeath/clutter,Distrotech/clutter,djdeath/clutter,djdeath/clutter,djdeath/clutter,GNOME/clutter,ebassi/clutter,Distrotech/clutter,Distrotech/clutter,djdeath/clutter,GNOME/clutter,Distrotech/clutter,jigpu/clutter,jigpu/clutter,Distrotech/clutter,ebassi/clutter,Distrotech/clutter,ebassi/clutter,GNOME/clutter,jigpu/clutter,ebassi/clutter,GNOME/clutter,GNOME/clutter,djdeath/clutter