imgutils: make systematic palette opaque.

Signed-off-by: NAnton Khirnov <anton@khirnov.net>

libavutil/imgutils.c

@@ -170,7 +170,7 @@ int avpriv_set_systematic_pal2(uint32_t pal[256], enum AVPixelFormat pix_fmt)
         default:
             return AVERROR(EINVAL);
         }
-        pal[i] = b + (g<<8) + (r<<16);
+        pal[i] = b + (g << 8) + (r << 16) + (0xFFU << 24);
     }
 
     return 0;