This eliminates the O(N^2) (N = map leaf count) operation of finding
visible lights and will later allow for finer culling of the lights as
they can be tested against the leaf volume (which they currently are
not as this was just getting things going). However, this has severely
hurt ad_tears' performance (I suspect due to the extreme number of
leafs), but the speed seems to be very steady. Hopefully, reconstructing
the vis clusters will help (I imagine it will help in many places, not
just lights).
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