Material assigned to meshes with different attributes

[lilleyse]

If a material is assigned to meshes with different attributes it should be duplicated. If one of those meshes doesn't have texture coordinates the textures in the duplicated material should be removed.

The glTF spec backs this up by saying:

Mesh must have corresponding texture coordinate attributes for the material to be applicable to it.

https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#textureinfotexcoord

Example obj snippet:

usemtl red
f 1//2 3//2 5//2 1//1
...
usemtl red
f 1/2/3 5/4/2 1/7/4