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