[F3D] glTF2 support

[Lilaa3]

This pr adds support for optionally exporting and importing f3d material data as glTF extensions, the objective is to create a standard that other tools can rely on (for example, tiny3d) instead of relying on the glTF addon's custom propreties toggle, along with implementing any needed hacks that broke exports previously. This pr is based on wiseguy's glTF64 but does not export the glTF64 extension.

Missing:

• Importing
• Emulate as much as possible within standard glTF2 extensions such as pbr metallic
• Warnings for invalid materials
• Schemas
• Vertex colors (create a hack for a single COLOR_0 acessor, tricky in different ways depending on version)
• Work around for texture gathering issues (node gather can´t handle f3d nodes)
• Make sure code is up to standard before review