Only create the tessface layer if it does not exist.

Use tangent generation code in game engine.

Tangents are now taken into account when generating tangents for
rendering - Normal maps work again.

Added special code that only creates a tessface layer for tangents, also
added code that creates tangent mface data from loop data during mesh
conversion.

Merge branch 'master' into temp-tangent-refactor

Conflicts:
source/blender/blenkernel/intern/DerivedMesh.c

Fix merge conflict caused by own smartness failing

Move display code and tangent calculation to loops WIP patch by Campbell

Convert tangent layer and baking to use looptris. This is WIP code
to make sure all blender uses tangents generated from loops.