DO NOT MERGE Update libpng to 1.6.20

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

gpc_sCp(Pixel *out, const Pixel *in, const Background *back)
{
 (void)back;

 if (in->a <= 128)
 {
      out->r = out->g = out->b = 255;
      out->a = 0;
 }

 else
 {
      out->r = sRGB((double)in->r / in->a);
      out->g = sRGB((double)in->g / in->a);
      out->b = sRGB((double)in->b / in->a);
      out->a = u8d(in->a / 257.);
 }
}

gpc_sCp(Pixel *out, const Pixel *in, const Background *back)
{
 (void)back;

 if (in->a <= 128)
 {
      out->r = out->g = out->b = 255;
      out->a = 0;
 }

 else
 {
      out->r = sRGB((double)in->r / in->a);
      out->g = sRGB((double)in->g / in->a);
      out->b = sRGB((double)in->b / in->a);
      out->a = u8d(in->a / 257.);
 }
}
