Stopped using ray differentials for now.

I was having trouble getting ray differentials right, so I've switched
to a much simpler (but far less robust) ad-hoc "ray width" approach.
This should help me move forward and work on other things.  In fact,
already it's helped me squash a couple of bugs... bugs that may have
actually been the problem, rather than ray differentials.

I'll definitely switch back to ray differentials at some point, probably
once I put in a proper shading system.  But for now I'll stick with the
simpler ad-hoc method.