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tcl funct: clear_selfintersect -- clear curr img
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keyboard equivalent: none

The "CLR" button (F3 panel lower middle) clears
the current 3D self intersect buffer (if loaded).

This is an internal 3D buffer updated from the
current state of the surface at the top of each
inner loop in shrink() if $chkselfintflag is set
to 1.

After the next proposed vertex movement has been
calculated (later in the shrink() loop), if the
movement would result in self-intersection
(penetration of the current self-intersect
volume), the normal force for that vertex is
zeroed.
