Miscellaneous fixes to catch up with the spec
Miscellaneous fixes to catch up with the spec

This change backfills all missing items for the elements from the spec
that we support (which is everything except for morph targets and
cameras atm).

Specifically:

- Textures and images now have names; they may contain important
semantic information or can be used for debugging.
- Accessors now have normalized attribute; without this it's impossible
to decode fixed point data correctly.
- Materials now have alpha mode and cutoff to be able to setup blending
correctly.
- Occlusion texture has "strength" instead of "scale"; we parse it into
"scale" field for simplicity.

jkuhlmann/cgltf,jkuhlmann/cgltf,jkuhlmann/cgltf