3D feature discussion

Nobody has claimed this yet.

This ticket is to track discussion on adding 3D features to the v3 spec.

* Likely the most compelling use case for 3D is for line features
* Polygons and points are less compelling:
* We represent 3D polygons in existing vector tiles with a single height property. For more complex 3D shapes it seems it would probably be better to use an existing spec (glTF) rather than reinvent the wheel within VTs.
* Supporting meshes would necessitate drastic retooling and probably won't work with our current feature model.
* 3D point data could likewise be easily represented with a scalar property

* Big question: what is the unit of the third dimension? One option would be to scale it to the other two dimensions' unit at a given YZ, though this would limit its extent to basically a vector cube.
* Big unknown: how this would affect tile size

No file or test is named; start by reviewing the v3 specification and the existing vector-tile feature model. The discussion mentions line features, scalar height properties, glTF, the unit of the third dimension, and tile size. This needs a settled design before implementation scope or completion criteria can be defined.

Written by the indexing model from the issue text.