DO NOT MERGE Update libpng to 1.6.20

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

u16d(double d)
{
   d = closestinteger(d);
 return (png_uint_16)d;
}

u16d(double d)
{
   d = closestinteger(d);
 return (png_uint_16)d;
}
