Decouple file I/O from .glb handling

[Kupoman]

In order to load gltf/glb data from memory As a use library based user I would like to have the File I/O decoupled from the .glb handling.

Right now I would need to re-implement the .glb handling if I want to load data from memory instead of a file. I have an example of what I am looking for here: https://github.com/Kupoman/panda3d-gltf/commit/0f358648e7bf8bae7fe543ba58e4d67b7ecbf480. If this looks like a reasonable approach, I can submit a PR. Otherwise, we can begin some design discussions here.