Relax external tileset restrictions

[lilleyse]

From the spec:

root.geometricError === tile.geometricError,
root.refine === tile.refine, and
root.boundingVolume === tile.content.boundingVolume (or root.boundingVolume === tile.boundingVolume when tile.content.boundingVolume is undefined).
root.viewerRequestVolume === tile.viewerRequestVolume or root.viewerRequestVolume is undefined.

Are these restrictions actually needed? This makes serving dynamic external tilesets harder.

[lilleyse]

@pjcozzi any thoughts here? Losing the restrictions should not present much of a problem for clients.

[pjcozzi]

The intention was to prevent senseless consistences and to semantically treat the tileset's root as the other tileset's leaf tile...all in the name of making the traversal implementation easier in theory.

If these restrictions do not help the implementation in practice and it would not be possible to make technically valid but problematic tilesets then let's relax the restrictions.

[ggetz]

Resolved in #301