GLTF: Move shared defines into a separate gltf_defines.h file

[aaronfranke]

In a nutshell, the current way the GLTF module works is like this:

• GLTFState knows about most of the small classes.
• GLTFDocument operates on GLTFState and the small classes.

As such, I found the current header situation confusing, since most of the small classes are including GLTFDocument. This PR fixes this confusing situation by moving the shared defines to a separate gltf_defines.h file, and moving the template conversion methods into a separate gltf_template_convert.h file. Now, there's a lot less including of gltf_document.h, and GLTFDocument declaring friend class is not necessary anymore.

[fire]

Approved. Mechanical move. As long as the compiler is happy :)

[akien-mga]

Thanks!