Clamped polygon boundingSphere issues

The Sandcastle below should zoom in on the polygon, and then tilt. Even though the entity is being tracked, it goes out of view.

[Sandcastle example.](https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/#c=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)

You can see `//height:0` is commented out. If you comment that back in, it works correctly, which is the expected behavior.

@likangning93 mentioned this was because clamped polygons use the center of the shadow volume, which is really tall in this case.

[Forum thread](https://groups.google.com/d/msg/cesium-dev/Ji977bmW-Qs/Hfp1YePHCgAJ).

Reproduce the linked Sandcastle example with the polygon's height line commented out, then compare its tracked zoom-and-tilt behavior when height:0 is restored. Read the linked forum thread and investigate the clamped polygon boundingSphere behavior, especially the shadow-volume center described there. Done means the tracked polygon remains in view without requiring height:0.

Written by the indexing model from the issue text.