When dealing with thin objects we really have two thicknesses to
consider, the thickness in the direction of the normal, which
corresponds to the value used for solid objects, and the thickness of
the object's walls, which generally is a constant.
Reusing thickness in the later case is problematic for assets that have
a thickness map, but are rendered hollow.
In the future we can even imagine handling double-sided hollow objects
by using the thickness information -- e.g. a hollow cube.
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