Create function to update tileset clipping plane matrix

[OmarShehata]

This is just a small refactor in Cesium3DTileset.js that creates a private function for updating the clippingPlaneOffsetMatrix so that other classes that use it can ensure that this matrix is up to date before using it.

@lilleyse

[cesium-concierge]

Thanks for the pull request @OmarShehata!

• ✔️ 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.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

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[OmarShehata]

Superseded by #7182