Construct a convex hull around a series of 2D outlines.
This is done be searching for planes perpendicular to the given
axis that contain voxels of color "bordercol". Whenever such a
plane is found, all bordercol voxels in that plane are
connected to the nearest voxel in the next found plane and
vice versa. The voxels along the connection lines are remarked
as the from and to parameters say.