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.
