Added workaround for broken tilesets.

[tfili]

A bunch of tilesets were generated that have a leading / in front of all URLs in the tileset.json. If the tiles aren't at the root of the domain they will not load anymore. If we find a b3dm file with a leading slash, we test load a tile. If it succeeds we continue on. If it fails, we set this to true so we know to strip the slash when loading tiles.

[cesium-concierge]

@tfili, thanks for the pull request! Maintainers, we have a signed CLA from @tfili, so you can review this at any time.

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[mramato]

Good idea @tfili, I can confirm this works and is probably the best solution we have for maintaining compatibility while allowed correct tilesets to work properly.

@lilleyse Can you please take a look at this to make sure you are okay with it? What should the tail on this be? One year? Forever? What has our plan been for other old things we've changed in the spec or otherwise had to fix?

@tfili if we decide to only keep this for a certain period of time, we should write an issue so we don't forget about it with whatever future date we think we should revisit this.

[mramato]

Talk to @lilleyse offline, he's good with this. He also suggested just removing it at the same time we handle #5363, so I added a comment there, no need to write up a new issue.