fmpz_mpolyl_gcd_brown, fmpz_mpolyl_gcd_brown_threaded_pool

Brown's dense multimodular algorithm [Bro1971] for the GCD and cofactors of nonzero $A$ and $B$. The integer contents are removed from $A$ and $B$, which are modified. Images are computed with nmod_mpolyn_gcd_brown_smprime modulo primes $p > 2^{\mathrm{FLINT\_BITS}-1}$ not dividing the GCD $\gamma$ of the leading coefficients, scaled to have leading coefficient $\gamma \bmod p$ and combined by Chinese remaindering, discarding images of too large degree. The loop stops when the product $m$ of the primes exceeds twice $\gamma$ times the larger height of $A$ and $B$ and the reconstructed $H$, $\bar A_H$, $\bar B_H$ satisfy $2 \min(\|H\|_\infty \|\bar A_H\|_1, \|H\|_1 \|\bar A_H\|_\infty) < m$ and likewise for $B$; this proves $H \bar A_H = \gamma A$ and $H \bar B_H = \gamma B$ without a division. Iv (possibly NULL) supplies degree bounds for the images. The threaded version computes images for different primes in parallel using the given thread pool handles; it is not used by fmpz_mpoly_gcd. Return 0 if the primes run out.