Blender Git Loki

Add tangent space computation/access from RNA (i.e. python).

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