Check buildPrepareG2

[paberr]

Hi,

I am not familiar with your code base, but when comparing the buildPrepareG2 with other pairing implementations, I found a difference in the following line:
https://github.com/iden3/websnark/blob/3dd30d54991610d1194cd78e140c4cd89edd3132/src/mnt6753/build_mnt6753.js#L438

Zexe's implementation that I am working with right now computes:
let minus_r_affine_x = r.x * &rz2_inv;
whereas your code multiplies by rz_inv.

I haven't checked all the code that is following, so perhaps your prepAddStep expects this as an input. However, I still wanted to bring up the difference for you to check.