Blender Git Loki

Fix (partial) T47198: Cycles have broken UVs in some degenerated quads cases

The issue was discontinuity in logic when importing vertices from blender
and then importing data layers regardless of how we split the face. Quite
interesting we didn't notice this issue before.

Thanks Bastien for the investigation, based on D1742 but redid it to make
patch a bit more clear to follow.