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function: fill_surface -- 3d fill current surface
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keyboard equiv: none

Performs a 3d fill on the current surface and
saved the result in an internal buffer.  It may
be saved with write_fill_images.

The fill begins by zeroing the 3D volume.

Then, any voxel that intersects a triangular or
quadrangular face of the surface is marked.

The volume is then filled up to the marked
surface-intersecting voxels.

The final step is to color the unfilled voxels
with $fillcolor3d and possibly also color the
surface-intersecting voxels, too, if the
$includeintersectingflag is set.

To use this volume to make a surface, name it (or
rename it) to filled.mgz and use SubjectTools ->
Create Orig Surface.
