DO NOT MERGE Update libpng to 1.6.20

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

image_transform_png_set_palette_to_rgb_set(PNG_CONST image_transform *this,
image_transform_png_set_palette_to_rgb_set(const image_transform *this,
     transform_display *that, png_structp pp, png_infop pi)
 {
    png_set_palette_to_rgb(pp);
 this->next->set(this->next, that, pp, pi);

 }

image_transform_png_set_palette_to_rgb_set(PNG_CONST image_transform *this,
     transform_display *that, png_structp pp, png_infop pi)
 {
    png_set_palette_to_rgb(pp);
 this->next->set(this->next, that, pp, pi);

 }
