DO NOT MERGE Update libpng to 1.6.20

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

 image_transform_png_set_strip_16_add(image_transform *this,
    const image_transform **that, png_byte colour_type, png_byte bit_depth)
 {
    UNUSED(colour_type)
 
 this->next = *that;
 *that = this;

 return bit_depth > 8;
}

 image_transform_png_set_strip_16_add(image_transform *this,
    PNG_CONST image_transform **that, png_byte colour_type, png_byte bit_depth)
 {
    UNUSED(colour_type)
 
 this->next = *that;
 *that = this;

 return bit_depth > 8;
}
