Blender Git Loki

Subdiv: Fix/Workaround for surface partial derivatives

Both partial derivatives were evaluated to 0 for a special vertex on
Suzanne model: this is happening on a vertex where two adjacent faces
with 2 common edges are connected (in the nose of Suzanne).

This was breaking multires in this point since tangent matrix is all
zeroes, and hence no displacement can be applied in that vertex.