background-content: Use redraw_clip when culling is unavailable

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.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1363

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