Houdini writes vertex data in a different format than Blender does; Houdini uses "face-varying scope", which means that the vertex colours are indexed by an ever-increasing number over all vertices of all faces instead of the vertex index. I've also merged the read_custom_data_mcols() and read_mcols() functions, because the latter was only called from the former, and the changes in this commit would add yet more function parameters to pass.