gltfpack: keep obj hierarchy and name

[kzhsw]

Motivation:
Currently gltfpack flattens and merges all obj meshes with the same material, so mesh hierarchy and name got lost even if -kn is passed.

Proposal:
With -kn argument, keep hierarchy and name from obj meshes, and do not merge it.
With -km argument, keep name from obj materials, and do not merge named obj materials.

Alternative:
obj2gltf keeps hierarchy and name by default.

[zeux]

What do you mean by hierarchy? .obj files don't have transforms, only groups, from what I understand.

[kzhsw]

What do you mean by hierarchy? .obj files don't have transforms, only groups, from what I understand.

Node name and hierarchy, like this:

Edit: It seems I got confused in mesh and primitive, just keeping mesh names is ok.

[zeux]

Right - I think the __root__ name here is synthesized either by obj2gltf or Babylon.js. The object and material names can be kept though.