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_bssrdf.cpp

@@ -105,5 +105,34 @@ ClosureParam *closure_bssrdf_gaussian_params()
 
 CCLOSURE_PREPARE(closure_bssrdf_gaussian_prepare, GaussianBSSRDFClosure)
 
+/* Burley */
+
+class BurleyBSSRDFClosure : public CBSSRDFClosure {
+public:
+	BurleyBSSRDFClosure()
+	{}
+
+	void setup()
+	{
+		sc.type = CLOSURE_BSSRDF_BURLEY_ID;
+		sc.data0 = fabsf(average(radius));
+	}
+};
+
+ClosureParam *closure_bssrdf_burley_params()
+{
+	static ClosureParam params[] = {
+		CLOSURE_FLOAT3_PARAM(BurleyBSSRDFClosure, sc.N),
+		CLOSURE_FLOAT3_PARAM(BurleyBSSRDFClosure, radius),
+		CLOSURE_FLOAT_PARAM(BurleyBSSRDFClosure, sc.data1),
+		CLOSURE_FLOAT3_PARAM(BurleyBSSRDFClosure, albedo),
+		CLOSURE_STRING_KEYPARAM(BurleyBSSRDFClosure, label, "label"),
+		CLOSURE_FINISH_PARAM(BurleyBSSRDFClosure)
+	};
+	return params;
+}
+
+CCLOSURE_PREPARE(closure_bssrdf_burley_prepare, BurleyBSSRDFClosure)
+
 CCL_NAMESPACE_END