Support for non-rectangular custom masks (polygons from shapely).

[PinkEyeSurvivor]

Thanks to the team for authoring this package!

I am requesting a feature that I believe will serve many people. I have WSI annotations in the form of shapely polygons which is likely to be a common end result of an annotation workflow. Would it be possible to make a custom mask function that can interact with Shapely Polygons directly?

In an ideal world I could supply a list of polygon vertices (at native resolution) and end up with a mask similar to the rectangular mask that you have already implemented.

Again many thanks!