Feature: DEXTR-based "Magic Segmentation"

[seveibar]

See https://youtu.be/6WJxzKsIFKA?t=2936

We can use an algorithm like DEXTR to do this quickly http://people.ee.ethz.ch/~cvlsegmentation/dextr/

Thanks to @Ownmarc for the research into implementation.

Ideally, there are two tools, one that outputs a pixel level segmentation #59, and one that outputs a polygon segmentation.