clamp for Mix node

the implementation was following my early commit for Math node
I haven't had a chance to run those through Brecht, but would like to do eventually. (they work fine though)

intern/cycles/render/nodes.h

@@ -329,6 +329,8 @@ class MixNode : public ShaderNode {
 public:
 	SHADER_NODE_CLASS(MixNode)
 
+	bool use_clamp;
+
 	ustring type;
 	static ShaderEnum type_enum;
 };