wumpus-drawing

Copperbox revision 2103.

I've re-implemented the LRText module to use the new PosImage type. The code ended up being more complicated than I expected, so it is perhaps not a significant improvement on the previous version. It has pointed to a limitation with Chains that was useful to spot now rather than later - its related to yesterday's problem where I'd misjudged whether a type should need access to the DrawingContext.