avformat: Add internal flags for AV(In|Out)putFormat
Both AVInputFormat and AVOutputFormat currently lack an equivalent to
AVCodec's caps_internal. E.g. if reading a header fails, each demuxer
is currently required to clean up manually, which often means to just
call the demuxer's read_close function. This could (and will) be done
generically via an equivalent of FF_CODEC_CAP_INIT_CLEANUP.
Because of the unholy ABI-relationship between libavdevice and
libavformat adding such a flag is only possible when the ABI is open
(despite the flag not being part of the public API), such as now.
Therefore such a flag is also added to AVOutputFormat, despite there
being no immediate use for it.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>