Strokes: Cannot be used to fill holes

[jlampel]

Strokes can be used to fill up to the boundary of a mesh, but it cannot be used to fill a hole in the middle of the mesh.

Given this scenario:

Drawing an I strip across the hole will leave the verts on the top and bottom disconnected:

While drawing an equals strip on the other edge leaves the middle verts of the side disconnected:

I'm not sure how important this feature is yet since we haven't talked about whether Patches should be fully ported or not (#1401). If we skip patches this becomes more important, but if we do add Patches, it's not really necessary at the moment.