modify IterativeLinearLSTriangulation function

modify IterativeLinearLSTriangulation function according 'Triangulation',  Hartley, R.I. and Sturm, P., Computer vision and image understanding, 1997

when the first loop, you get p2x and p2x1, but when the second loop, you get the same p2x and p2x1, so the loop will always exit when i=1.