Create new CesiumImage library for image-related code

[j9liu]

Description

This creates a new CesiumImage library to hold:

• ImageAsset
• ImageDecoder
• Ktx2TranscodeTargets
• ImageManipulation

This is some tech debt I ran into that prevented me from using functions from CesiumGltfContent in CesiumGltfReader. I figure I would rip the band-aid off and make this change since it's relatively straightforward.

Issue number or link

Closes #221

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

Testing plan

See:

• Update to image-library branch of cesium-native cesium-unreal#1852
• Use image-library branch of cesium-native cesium-unity#701

Reviewer checklist

Thank you for taking the time to review this PR. By approving a PR you are taking as much responsibility for these changes as the author.

As you review, please go through the checklist below:

• Review and run all parts of the test plan on this branch and verify it matches expectations.
  • If the issue is a bug please make sure you can reproduce the bug in the main branch and then checkout this branch to make sure it actually solved the issue.
• Review the code and make sure you do not have any remaining questions or concerns. You should understand the code change and the chosen approach. If you are not confident or have doubts about the code, please do not hesitate to ask questions.
  • Code should follow the C++ Style Guide
• Review the unit tests and make sure there are no missing tests or edge cases.
• Review documentation changes and updates to CHANGES.md to make sure they accurately cover the work in this PR.
• Verify that the Contributor License Agreement has been submitted, if needed.