Add support for vertex color attributes to AccessorUtility

[lilleyse]

Description

Adds support for vertex color attributes to AccessorUtility.

ColorFromAccessor always returns a glm::dvec4. If the underlying type is VEC3 the alpha channel is set to 1.0.

Issue number or link

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.

Remaining Tasks

Testing plan

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• 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.
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  • 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.
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[j9liu]

Thanks @lilleyse ! Looks great, I just have a few small comments before merge.

[lilleyse]

@j9liu Thanks! Ready for another review.

[j9liu]

Thanks @lilleyse !