has_decode_delay_been_guessed: tighten up the heuristic.

This adds the minimum delay needed with the current decoder to
recognize the reorder buffer size for the reference bitstreams.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -2292,7 +2292,10 @@ static int has_decode_delay_been_guessed(AVStream *st)
 #endif
     if(st->codec->has_b_frames<3)
         return st->info->nb_decoded_frames >= 6;
-    return st->info->nb_decoded_frames >= 20;
+    else if(st->codec->has_b_frames<4)
+        return st->info->nb_decoded_frames >= 18;
+    else
+        return st->info->nb_decoded_frames >= 20;
 }
 
 /* returns 1 or 0 if or if not decoded data was returned, or a negative error */