Fix EllipsoidGeometryUpdater crash for ellipsoid with a 0 radii component.

EllipsoidGeometryUpdater has a fast path that scales a unit sphere rather than recreate geometry every frame.  It was crashing due to a bad model matrix if the radii ever went to zero.  This change simply clamps values to 0.001, which visually has the same affect.
Merge pull request #1509 from AnalyticalGraphicsInc/ellipsoid-geometry

Fix EllipsoidGeometryUpdater crash for ellipsoid with a 0 radii component