Bake API: calculating differentials

Code with contribution from Brecht Van Lommel (he wrote the
bake_differentials actually ;)

Reviewers: brecht

Differential Revision: https://developer.blender.org/D510
Cycles-Bake: displacement support (fix T40068)

Code with contribution from Brecht Van Lommel.

Reviewers: brecht

Differential Revision: https://developer.blender.org/D510
ammend to [31cb7e39] Bake API: calculating differentials