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

intern/cycles/kernel/shaders/stdosl.h

@@ -510,6 +510,7 @@ closure color ambient_occlusion() BUILTIN;
 // BSSRDF
 closure color bssrdf_cubic(normal N, vector radius, float texture_blur, float sharpness) BUILTIN;
 closure color bssrdf_gaussian(normal N, vector radius, float texture_blur) BUILTIN;
+closure color bssrdf_burley(normal N, vector radius, float texture_blur, color albedo) BUILTIN;
 
 // Hair
 closure color hair_reflection(normal N, float roughnessu, float roughnessv, vector T, float offset) BUILTIN;