Wumpus

Copperbox revision 890.

I've changed the Picture type to make clipping more tangible - i.e. it should now be easier to account for clipping paths when considering the picture language operations (horizontal concatenation, etc.). PostScript output should support clipping, SVG output doesn't yet as I'll have to make it monadic to handle tagging with a counter for clip-path references.

I haven't done anything about the problems that yesterday's changes to the Picture type uncovered. I'm still thinking about the consequences.