[metapost] reverse clip operation

[Taco Hoekwater]

Andreas Matthias wrote:
> % now clipping the connection line
> path clip_p;
> clip_p := (-99u,-99u) -- (99u,-99u) -- (99u,99u) -- (-99u,99u) -- cycle
>    -- p -- cycle;   

Almost there:

   clip_p := (-99u,-99u) -- (99u,-99u) -- (99u,99u) -- (-99u,99u)
      -- p  --(-99u,99u)  -- cycle;


Be warned that the postscript interpreter will leave a single-pixel
line standing, though (because of the coincident paths).

Taco