DO NOT MERGE Update libpng to 1.6.20

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

modifier_color_encoding_is_sRGB(PNG_CONST png_modifier *pm)
modifier_color_encoding_is_sRGB(const png_modifier *pm)
 {
    return pm->current_encoding != 0 && pm->current_encoding == pm->encodings &&
       pm->current_encoding->gamma == pm->current_gamma;
 }

modifier_color_encoding_is_sRGB(PNG_CONST png_modifier *pm)
 {
    return pm->current_encoding != 0 && pm->current_encoding == pm->encodings &&
       pm->current_encoding->gamma == pm->current_gamma;
 }
