Comment 20 for bug 1760097

So this is my proposed 5.0 solution:

1) Keep the Fill Zones before DRC checkbox.
2) Keep its current default (unchecked).
3) When unchecked, re-calculate zone polygons and compare with existing polygons. If different, ask the user if they want to re-fill.

I've attached a patch which implements this for review.

I'm not intimately familiar with the zone fill code, so I'd appreciate other opinions on the amount of risk this involves.