Speedup up tangent space calculation
Speedup up tangent space calculation

This patch goes away form using C++ RNA during tangent space calculation which
avoids quite a bit of overhead. Now all calculation is done using data which
already exists in ccl::Mesh. This means, tangent space is now calculated from
triangles, which doesn't seem to be any different (at least as far as regression
tests are concerned).

One of the positive sides is that this change makes it possible to move tangent
space calculation from blender/ to render/ so we will have Cycles standalone
supporting tangent space.

Reviewers: brecht, lukasstockner97, campbellbarton

Differential Revision: https://developer.blender.org/D2810

tangent-opensource/coreBlackbird,tangent-opensource/coreBlackbird,tangent-opensource/coreBlackbird