The idea is to create vertices along the coarse edges once, without
splitting coarse edges on separate ptex faces. This requires some
indexing magic, vertices within a patch are no longer sequential.
Not sure how to make it nicer without such a black magic looking
calculations (which are basically boiling down to mimicking order
of verts/edges creation).
In the current offsets calculation loose verts and edges are not
properly taken into account, but those are causing topology refiner
to fail anyway, so it needs a bit deeper change.
Reviewers: brecht
Differential Revision: https://developer.blender.org/D3570
51c38b5d49