Overly restrictive assumption about ellipsoid radii being equal

There are a few places in the code that make the assumption that ellipsoids are "ellipsoids of revolution", meaning that `radii.x===radii.y` must hold.

Examples (not exhaustive):

- [`UniformState.js`](https://github.com/CesiumGS/cesium/blob/0a69f67b393ba194eefb7254600811c4b712ddc0/packages/engine/Source/Renderer/UniformState.js#L1298)
- [`Ellipsoid.js`](https://github.com/CesiumGS/cesium/blob/0a69f67b393ba194eefb7254600811c4b712ddc0/packages/engine/Source/Core/Ellipsoid.js#L704)
- ... (others might be added here...)

This assumption makes sense insofar that certain computations that are "just a bit tricky" for such ellipsoids can become _really_ complicated when the assumption can _not_ be made.

But given the recent changes that generalize aspects of the ellispoid handling (e.g. the `Ellipsoid.default`) and that allow users to essentially define "arbitrary" ellipsoids, it could make sense to review the places where the assumption about `radii.x===radii.y` is made, and think about whether or how this requirement could be relaxed.

(In some cases, it might even be possible to just _no longer pose this requirement_ - but the implications and effects on the lower-level ellipsoid computations should be thought through. Maybe one could get away with some `"Warning: [...] some computations may be imprecise"` here or there, but that remains to be investigated...)

Start by reviewing the cited locations in packages/engine/Source/Renderer/UniformState.js and packages/engine/Source/Core/Ellipsoid.js, then search for other assumptions that radii.x equals radii.y. Determine which computations can support arbitrary ellipsoids and which need a documented limitation or warning. Done means the affected cases and their supported behavior are identified and consistently handled.

Written by the indexing model from the issue text.