Improve ImageryLayer.cancelReprojections() handling

[kring]

Same as #9161 but in the main cesium repo, since I don't have commit access to @chris-cooper's branch. Also updates CHANGES.md.

This looks good to me, but someone else should review because I wrote it a year-ish ago.

We reproject web mercator imagery to geographic using a ComputeCommand. When the reproject command is created, we flip the imagery tile to the TRANSITIONING state. When the command completes, the state becomes READY. The problem is, when a new terrainProvider is set, cancelReprojections gets called, which simply throws away all pending imagery layer compute commands. If that happens after the reproject command is created but before it executes, then the imagery tile will never exit the TRANSITIONING state and any terrain tiles that its attached to will never be able to render.

So this PR extends a ComputeCommand with a canceled callback, and calls the callback before the command is thrown away. That way the imagery tile knows that it still needs to be reprojected later with a new command.

Fixes #6021

[cesium-concierge]

Thanks for the pull request @kring!

• ✔️ Signed CLA found.
• ❔ 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.

[mramato]

@IanLilleyT @lilleyse @likangning93 any chance one of you can review/merge this for the release tomorrow? (Only if it's the clear cut good change we think it is of course)

[lilleyse]

I can't seem to trigger the problem where half the globe is missing but I do occasionally see lower-res imagery get stuck, which is fixed on this branch.

[lilleyse]

I pushed a small style fix: 3f81df1