Bug: a88/11fa

Currently everything is plotted without magnification or reduction
on screen and scaled to paper size for print. But it looks as though
insufficient resolution on the canvas itself is producing aliasing
artifacts on the printouts -- e.g. slightly offset point fills on 
OS X, and slightly jagged confidence ellipses on equal-area plots.
Now that scaling is resolution-independent, it would be easy to 
maintain a very high resolution on the canvas and scale it down by
(say) 10 for plotting on the screen. (When zooming is implemented,
the zoom percentage would simply be scaled so that 100% was in fact
10%.)

This turns out on closer inspection to be not one issue but several.

1. Offset point fills seem to be an OS X bug; opened separately as #56.
2. Jagged confidence ellipses are a result of using java.awt.Polygon,
   which uses integer coordinates. I will try substituting GeneralPath
   which should fix it.
3. All the other drawing, as far as I know, uses float or double co-
   ordinates, so we shouldn't need to scale it up to get accuracy.

OK, fixed the jaggies with a GeneralPath (and fixed a printing bug in
passing), so closing this now.