Fix GEE terrain negative altitude handling

[mramato]

1. We were multiplying height by the negativeAltitudeExponentBias directly instead of -2^negativeAltitudeExponentBias which is correct in the comments, but not in the code.

2. The conversion from earth radius to meters needs to happen after the negative altitude bias is applied, otherwise the value would be incorrect.

[cesium-concierge]

Thanks for the pull request @mramato!

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Reviewers, don't forget to make sure that:

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

[mramato]

@tfili this needs to go out with the next release, but that's also your first day back in the office so you can just wait until that morning if you want. This is low impact outside of GEE.