gltf loader

[cgcostume]

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