There basically are two issues here: in smooth mode (and all non-tangent
normal map types) it doesn't invert the normal for backfacing polys;
on the other hand for flat shaded tangent type it is inverted too soon.
This fix does a brute force correction by checking the backfacing flag.
Reviewers: #cycles, brecht
Reviewed By: #cycles, brecht
Differential Revision: https://developer.blender.org/D2181
c376878e54