How should tiles point to external tilesets?

[lilleyse]

Right now a tile points to an external tileset with a content uri. But is this how we should do it in the future? Conceptually an external tileset is at the tile granularity not the content granularity. CC #583

{
  "asset": {
    "version": "1.0",
    "tilesetVersion": "1.2.3"
  },
  "geometricError": 240,
  "root": {
    "boundingVolume": {
      "region": [-1.31972, 0.69884, -1.31963, 0.69890, 0, 88]
    },
    "geometricError": 70,
    "refine": "ADD",
    "content": {
      "uri": "external.json"
    }
  }
}

[jerstlouis]

@lilleyse The confusion between content and tilesets was a great challenge for us in our implementation. It would be great to differentiate those better. See also CesiumGS/cesium#9356 .

With implicit tiling, there is actually no need for sub-tilesets at all with the approach I describe in #589.