When a lot of edges meet the same point - and, in particular, when

When a lot of edges meet the same point - and, in particular, when
they meet at sharp enough angles that the thick lines overlap
noticeably outside the circle of the point - redrawing the edges in a
different order each time looks ugly. Arrange to always redraw edges
in order of colour, so that empty lines are drawn before indeterminate
lines which in turn come before filled-in lines.

[originally from svn r9136]