Support non-spherical ellipsoid voxels

[jjhembd]

Our preliminary implementation of the ellipsoid voxel shape depended on a few simplifying assumptions. Some of these assumptions broke down for non-spherical ellipsoids, introducing inaccuracies in places like:

• Calculation of geodetic latitude.
• Computing heights.

The errors were small on a global scale for nearly-spherical ellipsoids (like WGS84). But at smaller scales (<10km) the errors could be significant.

This PR improves the accuracy of ellipsoid math in the voxel shaders. Along the way, some of the code was simplified to remove rarely-used or redundant shader defines and improve shader readability.

[cesium-concierge]

Thanks for the pull request @jjhembd!

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Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[jjhembd]

This mammoth PR has been superseded by several more focused PRs:

• Simplify voxel shaders #11838
• Compute normals for voxel shape intersections #11847
• Voxel ellipsoid math #11848