prevent colorToAlpha from carrying over from one imagery layer to its replacement

[likangning93]

#7727 had a funny bug where switching an imagery layer with color-to-alpha out and not setting color-to-alpha on the new layer would retain the color-to-alpha settings from the previous layer.

This PR always sets the value in the color-to-alpha uniform to a value that won't cause any change, regardless of whether it's used or not.

[cesium-concierge]

Thanks for the pull request @likangning93!

• ✔️ 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.

[likangning93]

Bug introduced in this release, so no CHANGES update.

[likangning93]

@bagnell there might be one more thing, will bump when ready.

[likangning93]

@bagnell ready!

[bagnell]

Thanks for catching these.