Godot GLTF import and export module

In a nutshell, the GLTF module works like this:

The structures/ folder contains GLTF structures, the small pieces that make up a GLTF file, represented as C++ classes.
The extensions/ folder contains GLTF extensions, which are optional features that build on top of the base GLTF spec.
GLTFState holds collections of structures and extensions.
GLTFDocument operates on GLTFState and its elements.