Add a public API for producing RFC 4281/6381 codecs trings for
MIME types.
This can be required for providing alternative video files to
a web browser, letting the browser pick the best file it supports.
Such strings also allow querying a browser whether it supports
a certain codec combination.
Finally, if implementing a DASH/HLS segmenter outside of libavformat,
one also has to generate such strings.
Generating such strings for H264/AAC is very simple, but for
more modern codecs, it can require a lot of nontrivial codec
specific parsing of extradata.
As libavformat already implements this, expose it for users as well.
The old, internal function ff_make_codec_str is kept and used by
the HLS and DASH muxers; the old function takes a logging context
which can be used for logging auxillary info about how the string
generation worked out.
The one in dashenc was added in
fe5e6e34c0, while the one in hlsenc
was added later in 0afa171f25. Both
have had various additions on top; merge both implementations
into one shared. (Notable additions in
060e74e2a9,
1cf2f040e3,
a2b1dd0ce3 and
797f0b27c175022d896e46db4ac2873e3e0a70af.)
For H264/avc1, use the implementation from hlsenc (which doesn't
use temporary allocations). For most other codecs, use the
only implementation from whichever had one.
The original dashenc implementation tried to be generic based
on RFC 6381, looking up codec tags in ff_codec_movvideo_tags
or ff_codec_movaudio_tags, and doing specific extra additions
for "mp4a" and "mp4v". In practice, only AV_CODEC_ID_AAC
and AV_CODEC_ID_MPEG4 ever mapped to these; simplify this to
a more straightforward codec id based handling, and merge
with the AAC profile based code from hlsenc.
There's a slight behaviour difference from the old one in
dashenc; if there's no code for a specific codec ID, we previously
just output what we matched from the mov tag tables, but now
we won't output anything. But most commonly used codecs in
DASH should be covered here.
