Experimented with partially mirroring some of the edge grid.  It did not
work very well though, as non-mirrored edges (in later pass) can ruin the
symmetry of the mirrored parts.
Mark boundary edges, show them in SVG image.
Worked on marking areas which lie on outside of boundary line.
fixed set_border() logic
Enable the mark_boundary_areas() logic, which is working now.