Use parent tilesets's tilesetVersion if it's set and none on the child.

[mramato]

If a parent contains a tilesetVersion parameter, then child tilesets should use it if they do not have a tilesetVersion of their own.

This fixes this demo where you can see it was dropping the v parameter for some tiles when you zoomed in.

@lilleyse are there any spec implications here? I couldn't find anything in the spec regarding tilesetVersion and external/child tilesets but perhaps it's an implementation guidelines instead of not in the spec itself?

[cesium-concierge]

Thanks for the pull request @mramato!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[lilleyse]

Looks good.

@lilleyse are there any spec implications here? I couldn't find anything in the spec regarding tilesetVersion and external/child tilesets but perhaps it's an implementation guidelines instead of not in the spec itself?

The wording isn't very clear about that. I'll open an issue. But this change seems more spec compliant since an external tileset is still part of the root tileset.

[lilleyse]

Opened CesiumGS/3d-tiles#381