wumpus-drawing

Copperbox revisions 2331 and 2332.

The new arrow tips now have all the functionality of the old tips, so I've removed the Wumpus.Drawing.Arrows modules.

Also I've added tip length to the the Arrow tip data type. To estimate good angles for tips on curved paths, you have to know the tip length (and halve it) rather than the retract distance. "Open" tips like barbs don't have a retract distance, so they were being drawn with the angle of the line end rather than the angle of the line at half the tip length - this doesn't look good on curved paths.