Add support for GLTF v2

[Tomas Polach]

older models (originally from vr.google.com domain) use GTLF v1 while newer models on poly.google.com use GLTF v2

examples:

• GLTF v1: https://poly.google.com/view/eW2LgmMV94J
• GLTF v2: https://poly.google.com/view/djxolbz_CYC

issue: A-Frame lib from v7 on supports only GLTF v2
solution:

1. load GTLF file
2. check GLF version
3. if required load legacy loader for GLTF v1