Fix OIT when toggling HDR.

[bagnell]

Fixes #7380. The OIT buffers need to be recreated when HDR is toggled.

I tried to write a test, but I couldn't get it to fail in master.

[cesium-concierge]

Thanks for the pull request @bagnell!

• ✔️ 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.

---

I am a bot who helps you make Cesium awesome! Contributions to my configuration are welcome.

🌍 🌎 🌏

[lilleyse]

Thanks @bagnell. Looks good.