clx: medium-clear-area: create a dynamic context for cleanup
clx: medium-clear-area: create a dynamic context for cleanup

When the background ink is a pattern then we need the context for
graphics, otherwise we end up in a debugger. We can't simply wrap it
in WITH-CLX-GRAPHICS, because then we use the foreground ink for GC.

slyrus/mcclim,slyrus/mcclim