Add support for ellipsoid shields

Currently, only spherical shields are supported. This leads to unwanted interactions between shields and bombers, where either the shield needs to have a tiny radius, or the bomber very high cruising altitude. Unfortunately, adjusting either of these will upset balance in other areas.

One possible workaround is to lower the shield in the y-axis and increase its radius, so that the portion above the ground is ellipsoid-like. However, this will create enormous unintended shielded areas if the shield generator is built at the top of a cliff or mountain.

Ellipsoid collision volumes are already supported, is it possible to extend this functionality to shields? The desire is only for a subset of ellipsoids - spheroids where x-axis = z-axis.

The issue names no files, tests, or entry points. Start by tracing existing shield collision handling and the ellipsoid collision-volume implementation; done means shields support spheroids with x-axis equal to z-axis without the unintended coverage described for elevated terrain.

Written by the indexing model from the issue text.