fftools/sync_queue: allow requesting a specific number of audio samples

This will be made useful in following commits.

fftools/sync_queue.h

@@ -71,6 +71,16 @@ int sq_add_stream(SyncQueue *sq, int limiting);
 void sq_limit_frames(SyncQueue *sq, unsigned int stream_idx,
                      uint64_t max_frames);
 
+/**
+ * Set a constant output audio frame size, in samples. Can only be used with
+ * SYNC_QUEUE_FRAMES queues and audio streams.
+ *
+ * All output frames will have exactly frame_samples audio samples, except
+ * possibly for the last one, which may have fewer.
+ */
+void sq_frame_samples(SyncQueue *sq, unsigned int stream_idx,
+                      int frame_samples);
+
 /**
  * Submit a frame for the stream with index stream_idx.
  *