DO NOT MERGE Update libpng to 1.6.20

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

gpc_nop6(Pixel *out, const Pixel *in, const Background *back)
{
 (void)back;
 if (in->a == 0)
      out->r = out->g = out->b = 65535;

 else
 {
      out->r = in->r;
      out->g = in->g;
      out->b = in->b;
 }

   out->a = in->a;
}

gpc_nop6(Pixel *out, const Pixel *in, const Background *back)
{
 (void)back;
 if (in->a == 0)
      out->r = out->g = out->b = 65535;

 else
 {
      out->r = in->r;
      out->g = in->g;
      out->b = in->b;
 }

   out->a = in->a;
}
