1) grow all soldermask relief by min_soldermask/2.
2) verify that all pads are in little islands of soldermask relief with
no other pads
Do we already check for too-small necks in polygon clearances? If so,
why not re-use that code?
we don't as far as I can tell. I just tried putting 2 vias very close
to each other in a rectangle of copper such that the remaining copper in
between was only about 1 mil wide. With a 5 mil min width DRC rule I
did not get a failure. Hmm, I also found another case which confuses
DRC. I drew a 1 mil wide line and then covered it with a 50 mil wide
line. DRC complains that I have a line which is too narrow.