DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)

ilineara_g22(int fixed_srgb, int alpha)
{
 return u16d((257 * alpha) * g22_to_d[fixed_srgb]);
}

ilineara_g22(int fixed_srgb, int alpha)
{
 return u16d((257 * alpha) * g22_to_d[fixed_srgb]);
}
