Prepare for templated functions.

Bug #915 is about an I/O exception thrown where none was allowed,
leading to crash via std::terminate().  However, instead of catching
and logging the error inside the decoder plugin, it should be able to
propagate the I/O error to the MPD core, so MPD can avoid trying other
decoder plugins.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/915

Closes https://github.com/MusicPlayerDaemon/MPD/issues/867

Will be needed for End Trimming (RFC7845 4.4,
https://github.com/MusicPlayerDaemon/MPD/issues/867).

Fixes memory leak.  That's what we get for

Fixes the first part of
https://github.com/MusicPlayerDaemon/MPD/issues/867

When the client wants to seek, but the decoder has already finished
decoding the current song, the player restarts the decoder with an
initial seek at the new position.  When this initial seek fails, MPD
pretends nothing has happened and plays this song from the start.

With this new flag, a restarted decoder marks the initial seek as
"essential" and fails the decoder if that seek fails.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/895

Fixes regression from commit 69f09648

The WildMidi project added the pkg-config file in version 0.3.3, but
unfortunately, Debian still doesn't ship it 4 years later:

 https://bugs.debian.org/916631

However, for cross-compiling, the pkg-config file is very helpful.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/833

This reverts commit eb192137.

This is no longer necessary because we require FFmpeg 3.1 or newer
since MPD 0.21.2.

This fixes a deprecation warning because the implicit AVPacket copy
constructor copies the deprecated attribute `convergence_duration`.

Found with readability-uppercase-literal-suffix
Signed-off-by: Rosen Penev <rosenp@gmail.com>

This is the case with uClibc-ng currently.
Signed-off-by: Rosen Penev <rosenp@gmail.com>
(cherry picked from commit 769cd0ee9f0cf8ceb026aa751b5d4a390bb5dbdc)
(changed define to match master)

Using libgme 0.6.2 on macOS, it appears that gme_info_t strings can be
empty, which creates weird track titles: (001/050)

This adds an additional check for an empty string.

Found with bugprone-suspicious-missing-comma
Signed-off-by: Rosen Penev <rosenp@gmail.com>

max_frames is size_t, not unsigned long. Fixes GCC warning.

When calling OggSeekFindEOS() from inside a OggVisitor callback, then
the #InputStream may be in the middle of an Ogg packet, and the newly
initialized #ogg_sync_state will not be able to load it without the
help of ogg_sync_pageseek().  By passing "synced=false" to
OggSeekFindEOS(), we force the use of ogg_sync_pageseek() even when
not actually seeking.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/719

Apparently, libmpcdec sets gain/peak variables to zero if they are not
present.  This clashes with our formula and results in bogus values
which cause noise during playback.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/640

The formula did not consider the channel count.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/639

Previously, a bogus value (whatever happened to be still in
`start_time`) was used.

See code comment.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/629