LAB-1658 Improve ImageryLayer.cancelReprojections() handling

[chris-cooper]

This was a bugfix that @kring helped us with a while back. From memory it was causing problems in an ImageryLayer when a terrain provider was swapped out on a globe.

[cesium-concierge]

Thanks for the pull request @chris-cooper!

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Reviewers, don't forget to make sure that:

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

[kring]

Thanks for opening this, @chris-cooper! I believe this fixes #6021. I'm going to push this into a branch in the main repo so make it easier to test.

[kring]

Confirmed I can't reproduce #6021 in this branch, but I can in master.

[chris-cooper]

Moved to #9163