Cycles: Implement approximate reflectance profiles

Cycles: Implement approximate reflectance profiles

Using this paper:

  http://graphics.pixar.com/library/ApproxBSSRDF/paper.pdf

This model gives less blurry results than the Cubic and Gaussian
we had implemented:

- Cubic: https://developer.blender.org/F279670
- Burley: https://developer.blender.org/F279671

The model is called "Christensen-Burley" in the interface, which
actually should be read as "Physically based" or "Realistic".

Reviewers: juicyfruit, dingto, lukasstockner97, brecht

Reviewed By: brecht, dingto

Subscribers: robocyte

Differential Revision: https://developer.blender.org/D1759