Update earcut to v2.2.1

[fredj]

According to the changelog, the performance for bigger inputs has been improved (5-12%).
See https://github.com/mapbox/earcut#changelog

[cesium-concierge]

Thanks for the pull request @fredj!

• ✔️ 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.
• ❔ Changes to third party files were made.
  • Looks like a file in one of our ThirdParty folders (ThirdParty/, Source/ThirdParty/) has been added or modified. Please verify that it has a section in LICENSE.md and that its license information is up to date with this new version.
• ❔ 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.

[mramato]

Thanks @fredj!