Revert "avformat: don't mark buffer as need to be writable, as we don't write into it directly"

This reverts commit 75890fb5.

This caused infinite loops in aviobuf when muxing to mkv.
write_flag seems to mean "mux" versus "demux".
The real meaning might be lost on us for lack of a libavformat documentation translator.

modules/demux/avformat/mux.c

@@ -120,7 +120,7 @@ int OpenMux( vlc_object_t *p_this )
 
     p_sys->io = avio_alloc_context(
         p_sys->io_buffer, p_sys->io_buffer_size,
-        0, p_mux, NULL, IOWrite, IOSeek );
+        1, p_mux, NULL, IOWrite, IOSeek );
 
     p_sys->oc->pb = p_sys->io;
     p_sys->oc->nb_streams = 0;