Ensure offCenterFrustum's values are updated

[ggetz]

There was a bug where calling the following functions in immediate succession would result in an DeveloperError: normalized result is not a number.

camera.switchToOrthographicFrustum();
viewer.flyTo(tileset);

As illustrated in this forum post.

This was because the values of _offCenterFrustum were not up to date. To remedy this, I made a getter for external use which calls update, and updated the external references to the property, ensuring they are called once to avoid running the update function multiple times (though it shouldn't be too much overhead regardless, which is why other getters do the same).

I also re-ordered the specs such that they should now fail if offCeneterFrustum values are not up to date.

[cesium-concierge]

Thanks for the pull request @ggetz!

• ✔️ Signed CLA found.
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  • 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.

[ggetz]

@jjhembd Conflicts here have been resolved and this is ready for review!

[jjhembd]

Thanks @ggetz! The changes look good, and it fixes the problem described in the forum post.