I started to change `EllipsoidPrimitive` to pass the depth range as a uniform, but it all seems kind of silly.  Like most primitives, `EllipsoidPrimitive` creates its own `RenderState`, and it doesn't specify a depth range.  So unless that changes, the depth range will be 0.0 to 1.0.  Is there any good reason not to just hard-code the depth range (with a comment perhaps) in the shader as well, with the assumption that if there's ever a good reason to change the depth range, it will be easy enough to change it in both places?  Less is more and all.  But tell me if I'm missing something.