The intuitive behavior when passing a selection to des.region, e.g.
local foo = selection.area(07,02,10,24)
des.region(foo, "lit")
is that foo will remain unmodified for further use. However, this wasn't
the case whenever making a lit region from it, because in order to light
walls adjacent to the lit area, the selection was having a grow
transformation applied as well. (This also seems like a problem - it
grows the selection even if what is being lit is not surrounded by
walls. I added a note in lua.adoc about this behavior.)
This fixes the selection mutation by cloning the passed-in selection and
growing the clone which leaves the original one unaffected.
This should not affect any special levels currently because the only
instance of des.region being used with a selection appears to be in
bigrm-2, which specifies *unlit* areas, which did not get grown.
69d37be878