wumpus-basic

Copperbox revision 1511.

I've added the dot / plot-mark drawing code from Wumpus-Extra and started updating it to use the GraphicF type rather than make Pictures for each dot.

Also, I've changed the rectangle drawing Basic.Graphic so the supplied point is the center rather than the bottom-left corner. Thus is the way TikZ does it, and as circles and shapes in Wumpus use the center it seems better to unify to that. textline is an exception as it takes the supplied point as baseline-left (matching PostScript's behaviour), I will keep it operating in this way but add another operation that makes text from the center.