lavf/mux: add the flush_packets option.

Note: a lot of muxers already do the flushing explicitly.

libavformat/avformat.h

@@ -1213,6 +1213,13 @@ typedef struct AVFormatContext {
      */
     int seek2any;
 
+    /**
+     * Flush the I/O context after each packet.
+     * - encoding: Set by user via AVOptions (NO direct access)
+     * - decoding: unused
+     */
+    int flush_packets;
+
     /*****************************************************************
      * All fields below this line are not part of the public API. They
      * may not be used outside of libavformat and can be changed and