av_demuxer_iterate() was added in libavformat 58.9.100.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1178

Found with cppcoreguidelines-special-member-functions
Signed-off-by: Rosen Penev <rosenp@gmail.com>

From libavformat/mov.c.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1173

This mapping was added 11 years ago in commit 766b9fd4, but FFmpeg
doesn't appear to support it.

These were added 11 years ago in commit 766b9fd4, but I cannot find
any evidence in the FFmpeg repository that these names were ever
supported.  This commit adds the tags as they are currently present in
libavformat/mov.c.

Upcoming release of game-music-emu will support it, details here: https://bitbucket.org/mpyne/game-music-emu/pull-requests/23/rsn-support

Replaces the static Duplicate() method.

To make things simpler, AllocatedString is now a non-template class.

Regression by commit a22d1c88

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1097

Replaces the static Duplicate() method.

To make things simpler, AllocatedString is now a non-template class.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1001

To be reused by the FFmpeg filter plugin.

Fixes regression by commit 23d5a2b8 -
that commit always pretended that any Opus file has both track and
album gain, and thus disabled the fallback to the other if one is not
set.

This patch changes the logic to only submit ReplayGain if at least one
value is set, and apply the offset only to that value.  If none is
available, then the new check in HandleAudio() will submit only the
output gain.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/977

"DEFAULT" is a bad name - all it says is that it's the default value,
but it doesn't say what it means.  The name NOTICE mimics the syslog
level.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/892