Red damage region

This is useful to visualize which parts of the screen are being damaged. Add a new 'damage-region' value for CLUTTER_PAINT and paint the damaged regions accordingly.

To use it, switch to another TTY and run:

$ env CLUTTER_PAINT=damage-region dbus-run-session -- gnome-shell --display-server --wayland

I'm not entirely sure whether it makes sense to paint both the clip AND the damage region. The native renderer for instance does not support redrawing regions.