The node would render black in this case (but should use the
'active_render' layer choosen in the object data properties -- this is
now in line to how this is handled for e.g. UVs)
This introduces ATTR_STD_VERTEX_COLOR and uses this thoughout, if no
particular layer is specified in the node.
Maniphest Tasks: T73938
Differential Revision: https://developer.blender.org/D6887
65ea5020c4