Solution for the "kissing corners" problem #281
Description
This is related to: https://www.klayout.de/forum/discussion/1250/width-space-special-drc
Two rectangles touching in one point form a polygon with self-touching points. This pinch is not recognized as a width violation. In the same way, two rectangles touching in one point can be kept as such using the "min_coherence=true" switch of the Region class, but a spacing check does not reveal the pinch as a spacing violation.
Solution is to handle this case by enhancing the "facing edges" criterion: edges with a distance of 0 are considered "facing each other" in the width and space sense if they have at least one point in common.
