GLTFExporter support

[qiangwai]

This loader works very well, I think the Three.GLTFExporter for node should be supported as well;
Especially when exporting GLTF models with images

[Brakebein]

I haven't been working yet with the three.js GLTFExporter in my projects. I just had a quick look into the code, and yes: the images/textures need to be handled differently.

Unfortunately, I cannot say when I will find time to implement this. Until then, any contribution is welcome.

[Brakebein]

It took a while, but I finally managed to adopt the GLTFExporter as well. It should work with simple textures. It won't work with materials with metalness and/or roughness, because it would require more complex changes. And I don't have an example use case for that.