Set maximum lowres value for the MJPEG decoder to 3.

While 4 works for some samples, 3 is the correct value since 8x8
DCT is used by (m)jpeg.

libavcodec/mjpegdec.c

@@ -1547,7 +1547,7 @@ AVCodec ff_mjpeg_decoder = {
     ff_mjpeg_decode_frame,
     CODEC_CAP_DR1,
     NULL,
-    .max_lowres = 4,
+    .max_lowres = 3,
     .long_name = NULL_IF_CONFIG_SMALL("MJPEG (Motion JPEG)"),
 };