Those were initially implemented and documented when I thought glTF
uses a full cone angle, and I forgot to update them once I discovered
glTF has a half-angle. This is thus now consistent with glTF defaults
again.
Otherwise the attenuation would explode to infinity at distance < 1,
which doesn't make any sense. This, together with the square of the
nominator, is different from the recommended glTF attenuation in
KHR_lights_punctial, but because I couldn't find any relevant discussion
on the equation used there, I'll assume it's just wrong.
Sigh.