add_thick_line = function(s, thickness)
	-- increase the thickness of a horizontal-ish line
	table.insert(Surface, s)
	for i=1,thickness-1 do
		s = copy_shape(s)
		for j=2,#s.data, 2 do
			s.data[j] = s.data[j]+1
		end
		table.insert(Surface, s)
	end
end