Add 3DTILES_layers

[lilleyse]

For CesiumGS/3d-tiles#863

The 3DTILES_layers extension is the successor to the 3D Tiles 1.1 groups concept which allows content to be associated with a particular layer, which may have application-specific properties.

At some point I would like to expand on the context and use cases but that might be in a future PR.

    {
      "extensions": {
        "3DTILES_tileset": {
          "geometricError": 0.0,
          "content": {
            "3DTILES_layers": {
              "layer": 0 // index into layers array in top level 3DTILES_layers extension
            }
          }
        }
      },
      "boundingVolume": {
        "shape": 1
      },
      "externalAsset": 0
    },

[weegeekps]

I'm guessing the goal here was to define most of the normative content through schema, but we need to define the properties in the normative text. (See §2.2.1.) That's the biggest issue I see right now. Anything else is minor.

[weegeekps]

Going forward we're supposed to ditch the allOf for single types. See this issue. This is one of the inclusions of glTF 2.0.2.

[weegeekps]

We probably need to expand on this a little. What are layers in the context of 3D Tiles? What are they used for? What (if any) limitations do they have? Etc.

We also need to normatively define all of the properties here. Schema alone isn't normative, but schema + normative text is.