wumpus-drawing

Copperbox revision 2287.

I've improved DocText so the passing of updates to the DrawingContext from the Doc evaluation is not so drastic - previously the whole DrawingContext was copied, or perhaps just shared but even so it seemed wrong.

Apropos the comment in the last post - I'm not sure that PosObject is wrong per se, but it does seem to be much less generally useful than I originally hoped.