fftools/ffmpeg: use the sync queues to handle -frames

Same issues apply to it as to -shortest.

Changes the results of the following tests:
- matroska-flac-extradata-update
  The test reencodes two input FLAC streams into three output FLAC
  streams. The last output stream is limited to 8 frames. The current
  code results in the first two output streams having 12 frames, after
  this commit all three streams have 8 frames and are the same length.
  This new result is better, since it is predictable.
- mkv-1242
  The test streamcopies one video and one audio stream, video is limited
  to 11 frames. The new result shortens the audio stream so that it is
  not longer than the video.

fftools/ffmpeg.c

@@ -3478,12 +3478,6 @@ static int need_output(void)
 
         if (ost->finished || of_finished(of))
             continue;
-        if (ost->frame_number >= ost->max_frames) {
-            int j;
-            for (j = 0; j < of->ctx->nb_streams; j++)
-                close_output_stream(output_streams[of->ost_index + j]);
-            continue;
-        }
 
         return 1;
     }