gltf loader

implement, or document / show examples for "raw" access on buffers/data loaded from gltf
I did not check with the current implementation but it should be possible to:
- for any scene object, access the buffers (WebGL handles/objects)
- access individual scene objects
- support use-case of custom rendering of scenes/geometries loaded using gltf loader
- support, e.g., instanced rendering (load single model, render multiple times...)
- we could add autom. instanced rendering for geometries that are references multiple times within the scene graph

Start with the current gltf loader and inspect how loaded scene objects, buffers, and geometries are represented. Check whether raw WebGL handles are already reachable and review any existing examples or API documentation. Done means documenting or implementing the requested access and custom or instanced-rendering use cases.

Written by the indexing model from the issue text.