Blender Git Loki

Subsurf: Rework in a way that patches boundaries are merged together

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