Avoid painting most wallpaper in the overview

When in the overview culling via self->clip_region is unavailable. The region is NULL because the paint call has not originated from a WindowGroup, because the overview does not use WindowGroup.

So the main wallpaper was being painted in full while in the overview. That's a waste of effort because redraw_clip is going to be used to stencil/scissor out only the parts that are changing. We don't need to paint most of the wallpaper, only the parts behind anything changing.

For the overview this reduces GPU power usage (intel_gpu_top) roughly 10% and reduces render times almost as much.