Cannot fly to entity clamped to ground in 2D

[Sandcastle](https://sandcastle.cesium.com/index.html#c=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). To reproduce:

* Switch to 2D mode
* Click "fly to entity"

Notice the camera flies to the wrong destination. Set the entity to a height reference of `NONE` and the camera will fly to the right location.

Reproduce the issue from the linked Sandcastle example by switching to 2D mode and choosing “fly to entity.” Compare the camera destination for an entity clamped to ground with the same entity using a height reference of NONE. Done means both configurations fly to the entity’s correct location in 2D, with a regression check for the clamped-to-ground case.

Written by the indexing model from the issue text.