-------------------------------------------
var,funct (left-click): read patch to mask surface
  set patch <infile>
  read_binary_patch_to_mask
-------------------------------------------

The "M" button on the "patch:" entry line reads
binary patch from file whose name is current
value of $patch (entry at left) in order to mask
current surface -- "mask" means, show current
surface vertices that are in the masking patch.

N.B.: The 3D coords in the patch are ignored so
any patch can be used as a mask.  For example, a
flat patch can be used to display the equivalent
part of a folded surface:

  1) read surface to be masked
  2) select patch to use as mask by holding down Shift
  3) click "M" button to mask current surface with patch

N.B.: holding down shift during selection blocks
the normal behavior of immediately autoloading
the patch after it is selected.

This procedure is useful in cases where you want
to make a cut on the simpler inflated surface,
but then apply the cut to the more complex folded
surface (where it would be impossible to specify
that cut because of self-occlusion).

An alternate way to perform that same operation is
to make a label from the patch (by filling the
entire patch), then use it to cut out the part of
the surface you want to keep (shift-middle-click
"R" button -- was "RC").

The "patch:" line method here avoids creating an
extra label file that might get stale if a patch
is re-made/modified.
