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/sync_queue.h

@@ -51,11 +51,13 @@ void       sq_free(SyncQueue **sq);
 /**
  * Add a new stream to the sync queue.
  *
+ * @param limiting whether the stream is limiting, i.e. no other stream can be
+ *                 longer than this one
  * @return
  * - a non-negative stream index on success
  * - a negative error code on error
  */
-int sq_add_stream(SyncQueue *sq);
+int sq_add_stream(SyncQueue *sq, int limiting);
 
 /**
  * Set the timebase for the stream with index stream_idx. Should be called
@@ -63,6 +65,13 @@ int sq_add_stream(SyncQueue *sq);
  */
 void sq_set_tb(SyncQueue *sq, unsigned int stream_idx, AVRational tb);
 
+/**
+ * Limit the number of output frames for stream with index stream_idx
+ * to max_frames.
+ */
+void sq_limit_frames(SyncQueue *sq, unsigned int stream_idx,
+                     uint64_t max_frames);
+
 /**
  * Submit a frame for the stream with index stream_idx.
  *