DO NOT MERGE Update libpng to 1.6.20

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

type_name(png_uint_32 type, FILE *out)
{
   putc(type_char(type >> 24), out);
   putc(type_char(type >> 16), out);
   putc(type_char(type >> 8), out);
   putc(type_char(type      ), out);
}

type_name(png_uint_32 type, FILE *out)
{
   putc(type_char(type >> 24), out);
   putc(type_char(type >> 16), out);
   putc(type_char(type >> 8), out);
   putc(type_char(type      ), out);
}
