set-clipping-region in dc<%>
Sets the clipping region for the drawing area, turning off all
clipping within the drawing region if #f is provided.
The clipping region must be reset after changing a dc<%>
object's origin or scale (unless it is #f); see
region% for more information.
See also
set-clipping-rect and
get-clipping-region.
( -> void
send a-dc set-clipping-region rgn)
rgn : region% object or #f