deduplicate textures when exporting gltf to godot project

[yankscally]

by hooking into the last gltf export step, there is way to attach the reference properly.

One of two things are happening:

1. the reference uses the same "link" that blender is using, from the Link/Append feature

2. the gltf export makes a brand new copy anyway, and the reference to the source file is discarded

I think it is 2, which means I will have to adapt the new reference.