Shading: Add Vertex Color node.

This patch adds a new Vertex Color node. The node also returns the alpha
of the vertex color layer as an output.

Reviewers: brecht

Differential Revision: https://developer.blender.org/D5767

release/scripts/startup/nodeitems_builtins.py

@@ -190,6 +190,7 @@ shader_node_categories = [
         NodeItem("ShaderNodeParticleInfo"),
         NodeItem("ShaderNodeCameraData"),
         NodeItem("ShaderNodeUVMap"),
+        NodeItem("ShaderNodeVertexColor"),
         NodeItem("ShaderNodeUVAlongStroke", poll=line_style_shader_nodes_poll),
         NodeItem("NodeGroupInput", poll=group_input_output_item_poll),
     ]),