omega/layout.py: honor the min/maj boundary parameters in linear boxes
Default to thicker lines in vector mode.

This is probably the better default. Might be worth adding more styles
for 'fine and precise' vs. 'robust' situations.
Default to painting RectPlot keys with a correct background

I've also finally added z-ordering to field painters. There are a couple of
subtleties that I ran into but it's generally actually quite simple.