This patch adds a new operator to convert a black and white image into
grease pencil strokes.
If the image is not B/W, an internal conversion is done.
This is the first operator using Potrace, but we expect to add more features in next Blender versions.
Reviewed By: HooglyBoogly
Maniphest Tasks: T79877
Differential Revision: https://developer.blender.org/D8951