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tickbox var: $chkfilledflag -- consider filled volume
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default: 0 (FALSE)

The "ckfill" tickbox (F3 panel, lower middle)
toggles whether a volume loaded using the "im2:"
entry (and corresponding "R" button) is consulted
during shrink operations.  This volume is
typically a standard "filled" volume.

If loaded+ticked, if a vertex is positioned
inside a non-zero (filled) voxel, the normal
force at this vertex is zeroed.

No effect of tick if "im2:" volume not loaded.
