Fix the issue where camera zoom get stuck when looking up

[baothientran]

Currently the camera zooming is stuck when looking up to the target. It is because the math for zooming assumes the camera is always looking down to the earth surface. The fix is to check the angle between the camera direction and the earth-origin-to-camera direction. If they are less than 120 degree, it looks up and we choose the method zoom-on-vector. Theoretically, we should check for 90 degree, but in practice, it doesn't work really well.

@lilleyse Can you please take a look at it? Please let me know if I should add anything

[cesium-concierge]

Thanks for the pull request @baothientran!

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[baothientran]

@lilleyse This is the sandcastle to replicate the problem. If you look up to the ceiling, it will get stuck in the master branch

[lilleyse]

@baothientran Nice fix! I can confirm this fixes the bug where the camera gets stuck when zooming towards a surface above it. This change also has the positive side effect of improving navigation at higher elevations (above 3000) because it now favors the zoomOnVector behavior except for bird's eye views. Zooming around Mt. Everest is less jolty than before.