GNUstep Cairo backend

After some effort, I finally managed to find a solution to the annoying scrolling bug in the Cairo backend... There's still some little graphic glitches appearing sometimes -- it looks like some clipping/refresh issues -- but overall it's nearly there :)

For the intrepid, the code is committed on the gnustep trunk. The above screenshot shows the Etoile desktop (with the new compositing manager) and the work-in-progress Narcissus theme, running with the Cairo backend.