Shader Node Editor: Add Closure to RGB convertion node.

Patch D3205 by Kanzaki Wataru

Only implemented in Eevee for now. Collapse a closure to RGBA so we can
do NPR stuff on the resulting color.

Use an emission shader to convert the color back to a closure.

Doing this will break PBR and will kill any SSR and SSS effects the shader
the shader rely on. That said screen space refraction and ambient occlusion
are supported due to the way they are implemented.

source/blender/nodes/NOD_shader.h

@@ -50,6 +50,7 @@ void register_node_type_sh_rgb(void);
 void register_node_type_sh_mix_rgb(void);
 void register_node_type_sh_valtorgb(void);
 void register_node_type_sh_rgbtobw(void);
+void register_node_type_sh_shadertorgb(void);
 void register_node_type_sh_normal(void);
 void register_node_type_sh_gamma(void);
 void register_node_type_sh_brightcontrast(void);