Experimental Godot implementation

[aaronfranke]

Hello! Even though glTFX is not yet finalized, I think it's great, so I wanted to implement it in Godot Engine.

This implementation is not to be considered a source of truth, and is subject to change whenever Khronos changes this repo.

Link: https://github.com/aaronfranke/godot-gltfx-reference-format (this is a plugin that you add to a project, requires Godot Engine 4.4 or later for full features).

Here are some trivial test files generated by my implementation: gltfx_test_files.zip

In order to make this, I mostly looked at the old glXF specification, but made the following assumptions:

• Since the format is called "glTF External Reference Format" instead of "glTF Experience Format", I assumed that "experience": true would be replaced by "reference": true.
• Since the format is now called "glTFX", I assumed that the file extension would be .gltfx.
• Since the existing examples do not include "scene" and "scenes", I did not do this either. See https://github.com/KhronosGroup/glTF/issues/1542 - multi-scene file formats aren't really a thing, and people have asked for this to be removed from glTF. I don't think glTFX should have this feature if it's being removed from glTF.
• I did not implement the "scene" property of asset. See https://github.com/KhronosGroup/glTF/issues/1542 - multi-scene file formats aren't really a thing, and people have asked for this to be removed from glTF. I don't think glTFX should encourage something that people consider a bad idea.