Cycles / Shader graph: Fallback to Sharp closures for very small roughness.

We fallback to Sharp closures for Glossy, Glass and Refraction nodes now, in case the Roughness input is disconnected and 0 (< 1e-4f to be exact).
This way we gain a few percentages of performance, in case the user did not manually set the closure type to "Sharp" in the UI.

Sharp will probably be removed from the UI as a followup, not needed anymore with this internal optimization.

Original idea by Lukas Stockner(Differential Revision: https://developer.blender.org/D1439), code implementation by myself.

intern/cycles/render/nodes.h

@@ -310,6 +310,8 @@ class GlossyBsdfNode : public BsdfNode {
 public:
 	SHADER_NODE_CLASS(GlossyBsdfNode)
 
+	void optimize();
+
 	ustring distribution;
 	static ShaderEnum distribution_enum;
 };
@@ -318,6 +320,8 @@ class GlassBsdfNode : public BsdfNode {
 public:
 	SHADER_NODE_CLASS(GlassBsdfNode)
 
+	void optimize();
+
 	ustring distribution;
 	static ShaderEnum distribution_enum;
 };
@@ -326,6 +330,8 @@ class RefractionBsdfNode : public BsdfNode {
 public:
 	SHADER_NODE_CLASS(RefractionBsdfNode)
 
+	void optimize();
+
 	ustring distribution;
 	static ShaderEnum distribution_enum;
 };