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
The implementation was also changed in a couple ways: use unsigned
integers as its base type rather than unsigned chars, and uses macros
rather than functions. (These could be changed to inline functions.)
Currently it is still only used during PBVH building, but now it's
accessible elsewhere.
