Storeboxes such as shelves do not let sunshine propagate because some of
the sides are opaque, but some sides are still open, and these should
still let light through.
During the optic scan phase, handle opaque nodes with visible inventory
by checking if the light travel direction also allows storebox access,
(from both the front and back face) and consider the node transparent in
such a case (unless, of course, the contained items are opaque).
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