Normalize cross product result for up vector to fix #6640

[xtassin]

At minima, normalizing the up vector obtained from cross product resolves issue #6640

Other cross product results in this function could also be normalized but only the up vector was causing the effect noticed in the mentioned issue.

This is my first pull request to Cesium, let me know if anything wrong.

[cesium-concierge]

Welcome to the Cesium community @xtassin!

Can you please send in a Contributor License Agreement (CLA) so that we can review and merge this pull request?

⚠️ I noticed that CHANGES.md has not been updated. If this change updates the public API in any way, fixes a bug, or makes any non-trivial update, please add a bullet point to CHANGES.md and comment on this pull request so we know it was updated. For more info, see the Pull Request Guidelines.

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[pjcozzi]

Thanks @xtassin, we received your CLA, someone will review soon!

[hpinkos]

Thanks @xtassin! I think we do also want to normalize the right vector.

And can you update CONTRIBUTORS.md in this pull request? You can edit the file right on GitHub here: https://github.com/xtassin/cesium/blob/patch-1/CONTRIBUTORS.md

[hpinkos]

@xtassin do you have time to finish up this pull request? We also want to normalize the right vector, then you should add yoursefl to the list in CONTRIBUTORS.md. Thanks!

[hpinkos]

Sorry for the delay on this @xtassin, I didn't realize you had committed those changes. Thanks for the contribution!