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variable: $insidesurffact -- rel brightness inside
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default: 0.60

The second unlabeled entry for "offset", which is
only visible on the larger F3 panel, controls the
relative brightness of the inside surface
(relative to the front surface).

No visible effect unless $lighttwosidedflag is
set (toggle with ctrl-middle-click on REDRAW)
*and* some part of inside surface is visible.

