How are extensions handled?

[pjcozzi]

Extensions in both tiles.json and the tiles themselves. We'll most likely take the same approach as glTF (for tiles.json) and quantized-mesh (for tiles).

Does tiles.json support both extensions and extras like glTF? Perhaps extra is useful for application-specific per-tile metadata that the app could process with a load event. We need concrete use cases.

[pjcozzi]

For now the schema has "additionalProperties" : false for all dictionary objects.

[ggetz]

Resolved in #301