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.
• The editor/ folder uses GLTFDocument to import and export 3D models.