clx: clipping pixmaps: provide sane optimizations
clx: clipping pixmaps: provide sane optimizations

- for polygon we draw the polygon
- for region-union we map over regions
- for region-difference we draw A with fg=1 and B wtih fg=0 (not tested)
- unoptimized path signals a simple-warning

We additionally fix a memory leak with lingering pixmaps (same strategy is used
as was implemented for gcontext-design with a special variable).

slyrus/mcclim,slyrus/mcclim