Invert Color Cycles Node

as with the HSV node the OSL code is relying on the (yet to be implemented) autorename.

Also the svm code could use mix (svm_lerp) instead:
 32 . float3 color_inv = make_float3(1.0f, 1.0f, 1.0f) - color;
 35 . . stack_store_float3(stack, out_color, svm_lerp(color_inv, color, factor));

I have a feeling that each node 'program' should have the least program as possible. I'll see with Brecht later.
But overall I don't know if that's any fast. And apart from that I think we will need this kind of function to move to a library if multiple functions linked in are not a problem.

intern/cycles/render/nodes.h

@@ -284,6 +284,11 @@ public:
 	SHADER_NODE_CLASS(MixClosureNode)
 };
 
+class InvertNode : public ShaderNode {
+public:
+	SHADER_NODE_CLASS(InvertNode)
+};
+
 class MixNode : public ShaderNode {
 public:
 	SHADER_NODE_CLASS(MixNode)