avformat/avformat: Deprecate AVFMT_ALLOW_FLUSH

It is of no value to the user, because every muxer can always
be flushed with a NULL packet. As its documentation shows
("If not set, the muxer will not receive a NULL packet in
the write_packet function") it is actually an internal flag
that has been publically exposed because there was no internal
flags field for output formats for a long time. But now there is
and so use it by replacing the public flag with a private one.

Reviewed-by: James Almer <jamrial@gmail.com>
Reviewed-by: Anton Khirnov <anton@khirnov.net>
Reviewed-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/mux.c

@@ -1206,7 +1206,13 @@ int av_write_frame(AVFormatContext *s, AVPacket *in)
     int ret;
 
     if (!in) {
+#if FF_API_ALLOW_FLUSH || LIBAVFORMAT_VERSION_MAJOR >= 61
+        // Hint: The pulse audio output device has this set,
+        // so we can't switch the check to FF_FMT_ALLOW_FLUSH immediately.
         if (s->oformat->flags & AVFMT_ALLOW_FLUSH) {
+#else
+        if (ffofmt(s->oformat)->flags_internal & FF_FMT_ALLOW_FLUSH) {
+#endif
             ret = ffofmt(s->oformat)->write_packet(s, NULL);
             flush_if_needed(s);
             if (ret >= 0 && s->pb && s->pb->error < 0)