backend: clx: %set-gc-clipping-region: issue warning
backend: clx: %set-gc-clipping-region: issue warning

This code never worked for areas not being rectangles nor region-sets
of them. Instead of ignoring this issue and acting as everyithing is
basically a rectangle-set, do nothing and issue a warning (for now).

slyrus/mcclim,slyrus/mcclim