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.

Eliminates a number of allocations, because callers don't need to copy
the strings to a newly allocated buffer only to null-terminate them.
And most callers don't need to have a null-terminated string.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Addresses #82.

Previously, the ffmpeg decoder only checked for the "generic"
MusicBrainz metadata keys used in other metadata container formats.

Don't scan tag items if the handler doesn't implement the tag()
method.

Don't scan tag items if the handler doesn't implement the tag()
method.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

This is the version present in Ubuntu Oneiric, the oldest distribution
with gcc 4.6.  Debian Squeeze is off target, because it has gcc 4.4,
which is unable to compile MPD anyway.

This commit drops all API compatibility hacks for older versions.

The new method pair() receives an arbitrary name/value pair.  Support
for this is being added to a few decoder plugins.

Pass a callback table to scan_file() and scan_stream(), instead of
returning a tag object.