Make Ellipsoids double-sided.

NOTE: This pull request targets `grid-material` where ellipsoid grids are a work in progress.

Have I committed any crimes here?  I didn't implement the idea of separate materials for inside & outside, mostly because that idea doesn't have practical application for me, but I could imagine a future pull request for it if there's external demand.  I just need both sides of the ellipsoid to render.  Have I done that correctly here?