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
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.