Commit b6d7cdd3cee9312156e20783248a3b12420b7a53 changed how the mesh data
is deformed, which wasn't taken into account yet in this unit test.
Instead of directly reading the mesh vertices (which aren't animated any
more), we convert the modified mesh to a new one, and inspect those
vertices instead.
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.
