The webm_chunk muxer caches its output to a dynamic buffer and when it
outputs anything, it explicitly flushes it. So set the flags indicating
that flushing after each packet should not be done automatically
(basically avoiding avio_write_marker() to be called by flush_if_needed()
in libavformat/mux.c).
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>