add translucency to outlineColor

[LHolst]

fix for #8553

PR adds translucency to outlineColor in BillboardCollectionVS

[cesium-concierge]

Thank you so much for the pull request @LHolst! I noticed this is your first pull request and I wanted to say welcome to the Cesium community!

The Pull Request Guidelines is a handy reference for making sure your PR gets accepted quickly, so make sure to skim that.

• ❌ Missing CONTRIBUTORS.md entry.
  • Please add yourself to the contributors file!
• ❌ Missing CLA.
  • Please send in a Contributor License Agreement (CLA) and comment back here to let us know to check this!
• ❔ 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.

[OmarShehata]

Thanks @LHolst ! Can you please fill out a contributor's license agreement? https://github.com/CesiumGS/cesium/blob/master/CONTRIBUTING.md#contributor-license-agreement-cla

[LHolst]

CLA signed and CONTRIBUTORS updated

[hpinkos]

Thanks @LHolst! I'll try to review this and get it in before the next release

[hpinkos]

Looks great @LHolst, thanks! I'll merge this as soon as CI passes

[LHolst]

Thanks @hpinkos !