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function: cut_plane -- cut surface using a plane
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keyboard equivalent: none

The "PLANE" button (lower right) cuts through the
surface using a plane and then saves only those
vertices on one side of the plane.

First, clear all marks with a R-click in the
surface display window.

Then select 4 surface vertices.  The first 3
define a plane to use for cutting, and the last
marked vertex indicates which part of the surface
(which side of the plane) should be saved.  If
more than 4 vertices have been selected, last 4
are used.

This automatically turns on $lighttwosidedflag so
the inside surface is visible.  The second
unlabeled entry for "offset", visible on the
larger F3 panel, controls the relative brightness
of the inside surface (relative to front surface)
using the tcl variable $insidesurffact.

For reasons obscure, the change in the lighting
model requires calling do_lighting_model twice to
make the inside surface appear gray (otherwise
it will appear cyan):

  do_lighting_model
  do_lighting_model
  redraw

This is automatically done by the PLANE button.

