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_test.c

@@ -147,6 +147,11 @@ const FFOutputFormat ff_fifo_test_muxer = {
     .write_trailer  = failing_write_trailer,
     .deinit         = failing_deinit,
     .p.priv_class   = &failing_muxer_class,
+#if FF_API_ALLOW_FLUSH
     .p.flags        = AVFMT_NOFILE | AVFMT_ALLOW_FLUSH,
+#else
+    .p.flags        = AVFMT_NOFILE,
+#endif
+    .flags_internal = FF_FMT_ALLOW_FLUSH,
 };