Cycles: per-BSDF normal input and new Bump node.

Each BSDF node now has a Normal input, which can be used to set a custom normal
for the BSDF, for example if you want to have only bump on one of the layers in
a multilayer material.

The Bump node can be used to generate a normal from a scalar value, the same as
what happens when you connect a scalar value to the displacement output.

Documentation has been updated with the latest changes:
http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/Nodes

Patch by Agustin Benavidez, some implementation tweaks by me.

intern/cycles/render/graph.h

@@ -235,6 +235,7 @@ protected:
 	void break_cycles(ShaderNode *node, vector<bool>& visited, vector<bool>& on_stack);
 	void clean();
 	void bump_from_displacement();
+	void refine_bump_nodes();
 	void default_inputs(bool do_osl);
 };