Problem

The gapcolor property allows drawing "stripey" Line2Ds and LineCollections with two different colors. Another set of artists that could benefit from this feature is patches -- currently, their edge can be drawn with dashes, but not with two different colors.

An example use case is to draw (unfilled) patches on a background whose color is not controlled, and can in particular be very light or very dark. In that case, using stripey patches alternating black and white ensures that the patch is visible no matter the underlying background.

Proposed solution

Add support for gapcolor in Patches. (Maybe the property should be named "edgegapcolor" for consistency.)