Merge some duplicate code.

Since AVFilterContext are freed automatically, this wrapper class
serves no purpose.  Let's remove it.

Fixes potential double-free bugs which currently did not occur because
the destructors happened to be called in the right order.

Fixes part 1 of https://github.com/MusicPlayerDaemon/MPD/issues/1235

Prevent a segfault when accessing album art.

Fix #1224 #1225

This replaces 967af603 with a more
effective workaround.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/822

snd_mixer_handle_events() has already been called by
DispatchSockets().  This way, we can also skip the exception handler.

The "install_dev" target runs ranlib during installation, and this
can break the Android build.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1019

Prepending "../" to absolute URIs would break them.

If UpdateStreamTag() gets called while an initial seek is pending, the
result will never be submitted to a MusicChunk.  By avoiding the
UpdateStreamTag() call in that case (by moving UpdateStreamTag() to
after the PrepareInitialSeek() check), the song_tag is preserved until
UpdateStreamTag() is called again from SubmitData().

This fixes missing tags in the "httpd" output.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1137

We need this even when AFMT_S24_PACKED is not available, for the
correct channel order in multi-channel files.  Internally, MPD uses
FLAC channel order, but OSS uses the same channel order as ALSA.

This way, we save the overhead for acquiring the tag_pool_lock.

Also, use RemoveAll() instead of directly clearing TagBuilder::items in
most cases, as its elements represent references that must be released.

Closes #1023

This plugin has been defunct for several years.  Tidal has not ever
replied to any of my emails, so they're apparently not interested in
MPD support.

ProxyInputStream::Read() assigns the `offset` field, which is the
wrong offset because it does not consider Icy metadata removed from
the stream.  Therefore, after every ProxyInputStream::Read() call,
IcyInputStream::Read() needs to override this offset.  This was
missing at the end of the stream, when Read()==0.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1216

this->OwnsTag() accesses fields that are not yet initialized.

Instead of checking for "cover.jpg" in the virtual directory
representing the CUE sheet, check its enclosing directory.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1206

Updating the database no longer requires the `admin` permission, only
`control` is needed (changed in 2abad0f4).

See also: #1124