Cycles: random walk subsurface scattering.

It is basically brute force volume scattering within the mesh, but part
of the SSS code for faster performance. The main difference with actual
volume scattering is that we assume the boundaries are diffuse and that
all lighting is coming through this boundary from outside the volume.

This gives much more accurate results for thin features and low density.
Some challenges remain however:

* Significantly more noisy than BSSRDF. Adding Dwivedi sampling may help
  here, but it's unclear still how much it helps in real world cases.
* Due to this being a volumetric method, geometry like eyes or mouth can
  darken the skin on the outside. We may be able to reduce this effect,
  or users can compensate for it by reducing the scattering radius in
  such areas.
* Sharp corners are quite bright. This matches actual volume rendering
  and results in some other renderers, but maybe not so much real world
  objects.

Differential Revision: https://developer.blender.org/D3054

intern/cycles/kernel/svm/svm_closure.h

@@ -764,7 +764,8 @@ ccl_device void svm_node_closure_bsdf(KernelGlobals *kg, ShaderData *sd, float *
 #ifdef __SUBSURFACE__
 		case CLOSURE_BSSRDF_CUBIC_ID:
 		case CLOSURE_BSSRDF_GAUSSIAN_ID:
-		case CLOSURE_BSSRDF_BURLEY_ID: {
+		case CLOSURE_BSSRDF_BURLEY_ID:
+		case CLOSURE_BSSRDF_RANDOM_WALK_ID: {
 			float3 weight = sd->svm_closure_weight * mix_weight;
 			Bssrdf *bssrdf = bssrdf_alloc(sd, weight);