DO NOT MERGE Update libpng to 1.6.20

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

image_transform_png_set_expand_mod(PNG_CONST image_transform *this,
image_transform_png_set_expand_mod(const image_transform *this,
     image_pixel *that, png_const_structp pp,
    const transform_display *display)
 {
    /* The general expand case depends on what the colour type is: */
    if (that->colour_type == PNG_COLOR_TYPE_PALETTE)
      image_pixel_convert_PLTE(that);
 else if (that->bit_depth < 8) /* grayscale */

       that->sample_depth = that->bit_depth = 8;
 
    if (that->have_tRNS)
      image_pixel_add_alpha(that, &display->this, 0/*!for background*/);
 
    this->next->mod(this->next, that, pp, display);
 }

image_transform_png_set_expand_mod(PNG_CONST image_transform *this,
     image_pixel *that, png_const_structp pp,
    PNG_CONST transform_display *display)
 {
    /* The general expand case depends on what the colour type is: */
    if (that->colour_type == PNG_COLOR_TYPE_PALETTE)
      image_pixel_convert_PLTE(that);
 else if (that->bit_depth < 8) /* grayscale */

       that->sample_depth = that->bit_depth = 8;
 
    if (that->have_tRNS)
      image_pixel_add_alpha(that, &display->this);
 
    this->next->mod(this->next, that, pp, display);
 }
