3D Tiles Serializer

[IanLilleyT]

There should be a 3D Tiles serializer for Cesium Native that goes between C++ and JSON. It would function very similarly to the glTF serializer and could probably use the same JSON schema <-> C++ generator located here, but with a few tweaks, as brought up in #270. Note that right now only the glTF reader is generated automatically, not the writer (see #228 for generating glTF writer).

The 3D Tiles serializer would not be responsible for pnts, b3dm, glTF, or any other external buffer data. It would only do the tileset.json. One challenge will be external tilesets. For now, we could keep it simple and let the caller create the external tilesets themselves (possibly using the 3D Tiles serializer).

Ultimately we'll want this to work with 3D Tiles exensions too, similar to how extensions are done in the glTF, but getting it working with the base 3D Tiles schema is a good start. I attached a small sample tileset below.

Resources:
3D Tiles schemas: https://github.com/CesiumGS/3d-tiles/tree/master/specification/schema
3D Tiles extensions: https://github.com/CesiumGS/3d-tiles/blob/3d-tiles-next/3D_TILES_NEXT.md
3D Tiles Metadata extension: https://github.com/CesiumGS/3d-tiles/tree/3d-tiles-next/extensions/3DTILES_metadata/1.0.0/schema
Sample data: sign.zip

Localhost sandcastle for viewing sample data

CC @baothientran @kring

[krupkad]

@lilleyse
Current status of this work (as of Aug 11):

• "Groundwork" is merged (Extract extension handling #299, Rename Cesium3DTiles -> Cesium3DTilesSelection #301)
• Writer is under review (3d tiles writer #306)
• Reader PR is open (3d tiles reader #308)

[kring]

This has been implemented.