Cycles: add Principled Hair BSDF.

This is a physically-based, easy-to-use shader for rendering hair and fur,
with controls for melanin, roughness and randomization.

Based on the paper "A Practical and Controllable Hair and Fur Model for
Production Path Tracing".

Implemented by Leonardo E. Segovia and Lukas Stockner, part of Google
Summer of Code 2018.

release/scripts/startup/nodeitems_builtins.py

@@ -246,6 +246,7 @@ shader_node_categories = [
         NodeItem("ShaderNodeVolumeAbsorption"),
         NodeItem("ShaderNodeVolumeScatter"),
         NodeItem("ShaderNodeVolumePrincipled"),
+        NodeItem("ShaderNodeBsdfHairPrincipled", poll=object_shader_nodes_poll)
     ]),
     ShaderNewNodeCategory("SH_NEW_TEXTURE", "Texture", items=[
         NodeItem("ShaderNodeTexImage"),