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/osl/osl_closures.cpp

@@ -236,6 +236,8 @@ void OSLShader::register_closures(OSLShadingSystem *ss_)
 		closure_bssrdf_cubic_params(), closure_bssrdf_cubic_prepare);
 	register_closure(ss, "bssrdf_gaussian", id++,
 		closure_bssrdf_gaussian_params(), closure_bssrdf_gaussian_prepare);
+	register_closure(ss, "bssrdf_burley", id++,
+		closure_bssrdf_burley_params(), closure_bssrdf_burley_prepare);
 
 	register_closure(ss, "hair_reflection", id++,
 		bsdf_hair_reflection_params(), bsdf_hair_reflection_prepare);