This simply mimics code used for loopnormals, to enable py scripts to generate and access (temporary)
a tangent 3D vector and bitangent sign for each loop. Together with the split normals, this allow
to recreate a complete tangent space for normal mapping (bitangent = bitangent_sign * cross(normal, tangent)).
Expects all faces to be tri or quads.
Reviewed By: Brecht, campbellbarton
Differential Revision: https://developer.blender.org/D185
8952f58375