Override texture flags not having any effect

Hey, I have an OBJ model without an MTL file. So I'm trying to pass my textures via the external override flags, for example: `obj2gltf -i mymodel.obj -o mymodel.gltf --baseColorTexture=assets/mymodel_albedo.png`.

However, this has no effect, the generated gltf file is the same as the model that gets generated when I remove the `--baseColorTexture` flag.

I also saw #141 and tried to create an empty MTL file and add `mtllib mymodel.mtl` as the first line to my OBJ file, which didn't change anything unfortunately. Am I using these flags wrong?

Start by reproducing the reported obj2gltf command with an OBJ that has no MTL file, then trace how the --baseColorTexture override is handled. Done means the generated glTF uses the supplied texture, and the behavior is covered by an appropriate regression test if the repository has one.

Written by the indexing model from the issue text.