DO NOT MERGE Update libpng to 1.6.20

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

standard_palette_init(standard_display *dp)
{
   store_palette_entry *palette = store_current_palette(dp->ps, &dp->npalette);

 /* The remaining entries remain white/opaque. */
 if (dp->npalette > 0)
 {
 int i = dp->npalette;
      memcpy(dp->palette, palette, i * sizeof *palette);

 /* Check for a non-opaque palette entry: */
 while (--i >= 0)
 if (palette[i].alpha < 255)
 break;

#     ifdef __GNUC__
 /* GCC can't handle the more obviously optimizable version. */
 if (i >= 0)
            dp->is_transparent = 1;
 else
            dp->is_transparent = 0;
#     else
         dp->is_transparent = (i >= 0);
#     endif
 }
}

standard_palette_init(standard_display *dp)
{
   store_palette_entry *palette = store_current_palette(dp->ps, &dp->npalette);

 /* The remaining entries remain white/opaque. */
 if (dp->npalette > 0)
 {
 int i = dp->npalette;
      memcpy(dp->palette, palette, i * sizeof *palette);

 /* Check for a non-opaque palette entry: */
 while (--i >= 0)
 if (palette[i].alpha < 255)
 break;

#     ifdef __GNUC__
 /* GCC can't handle the more obviously optimizable version. */
 if (i >= 0)
            dp->is_transparent = 1;
 else
            dp->is_transparent = 0;
#     else
         dp->is_transparent = (i >= 0);
#     endif
 }
}
