Support custom ellipsoids in Cesium3DTile bounding volume calculations

[jjhembd]

Feature

Bounding volume calculations in Cesium3DTile currently assume the default WGS84 ellipsoid. These should instead use the ellipsoid associated with the Globe or frameState.mapProjection in case the user has specified a custom ellipsoid.

For tiles with region type bounding volumes, the custom ellipsoid will need to be made available to the Cesium3DTile constructor, which currently does not have access to frameState.

[javagl]

Maybe someone can quickly confirm whether this is obsolete with the recent generalizations from https://github.com/CesiumGS/cesium/pull/12000 - I think that most places that assumed WGS84 should have changed to use the one that was configured to be the "default" one, right?

[ggetz]

For Cesium3DTile, the WGS84 ellipsoid is specifically used throughout when creating the region bounding volumes. However, based on the spec for regions, I don't think this should change when the scene's ellipsoid changes.

I'll close, but please let me know if anyone disagrees.