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/fifo.c

@@ -715,11 +715,16 @@ const FFOutputFormat ff_fifo_muxer = {
     .p.name         = "fifo",
     .p.long_name    = NULL_IF_CONFIG_SMALL("FIFO queue pseudo-muxer"),
     .p.priv_class   = &fifo_muxer_class,
+#if FF_API_ALLOW_FLUSH
     .p.flags        = AVFMT_NOFILE | AVFMT_ALLOW_FLUSH | AVFMT_TS_NEGATIVE,
+#else
+    .p.flags        = AVFMT_NOFILE | AVFMT_TS_NEGATIVE,
+#endif
     .priv_data_size = sizeof(FifoContext),
     .init           = fifo_init,
     .write_header   = fifo_write_header,
     .write_packet   = fifo_write_packet,
     .write_trailer  = fifo_write_trailer,
     .deinit         = fifo_deinit,
+    .flags_internal = FF_FMT_ALLOW_FLUSH,
 };