source 2007 Meloni "New point addition formulae for ECC applications", page 192
assume Z1 = Z2
compute A = (X2-X1)^2
compute B = X1 A
compute C = X2 A
compute D = (Y2-Y1)^2
compute X3 = D-B-C
compute Y3 = (Y2-Y1)(B-X3)-Y1(C-B)
compute Z3 = Z1(X2-X1)