Async pick explicit rendering fix

[lilleyse]

Fixes #8281

I could not trigger the problem by modifying the existing clamping demo but this is still a good fix regardless. More details in #8281.

Sandcastle

[cesium-concierge]

Thanks for the pull request @lilleyse!

• ✔️ 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.

[IanLilleyT]

Looks good, thanks @lilleyse!