Structural Metadata: answer todos

[gkjohnson]

Open Questions

• Is it the case that all textures are guaranteed to be uint8 type?
• Recommendations on handling BigInt?
• Are property tables usable without node property and / or mesh features?
• Are attribute accessors usable without non point objects?
• What are the usable cases when an internal node does and doesn't contain an extension reference?

TODO

• Expose used texture from mesh features info and property textures