computeFlyToLocationForRectangle needs to wait for terrain to become …

[mramato]

…ready

If you performed a geocode before the terrain became ready, it would throw an exception.

Fixes #6908.

Review this with whitespace diffing off, this PR is 99% indentation changes.

CC @lilleyse

[cesium-concierge]

Thanks for the pull request @mramato!

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[lilleyse]

Thanks, the fix works great.