6952 - raytracing: much better

The image is now visually indistinguishable from the baseline, though the
file isn't quite bit-for-bit correct.

I found 3 bugs:
a) I forgot to normalize the ray. After creating a helper to "automatically"
do it for me, it turns out said helper requires manually using.
b) I forgot to multiply by t at one place.
c) vec3-length was half-written.

For the umpteenth time, the bugs were all in the last place I looked. I
was worried about spending a lot of time transcribing `main` without any
feedback, but that turned out to be perfect.

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