Eliminate more duplicate code.

Don't bother to check for ADIF just because the stream is not
seekable.

Merge some duplicate code.

Eliminate some overhead when the caller doesn't need the buffer.

Checking the integer is faster, easier and more reliable.

.. instead of declaring a new constant.

Replaces the loop in sndfile_vio_read(), eliminating duplicate and
fragile code.

.. instead of InputStream::LockRead(). The former is cancellable.

Implement a "bulk" edit mode that postpones both UpdateQueuedSong()
and OnModified().  This way, the playlist version gets incremented
only once.  More importantly: when adding multiple songs to a queue
that consists of only one song, the first song that got added will
always be played next.  By postponing this choice, all newly added
songs get a chance to become the next song.  Fixes the second (and
last) part of Mantis ticket 0004005.

Don't restore the current song after shufflung when MPD is stopped
(but still remembers the current song internally).  Fixes the first
part of Mantis ticket 0004005.

Addresses Mantis ticket 0004015.

[mk: use decoder_read_full() only when needed, and a few formal
changes]

Works around WAV stream playback bug, because libaudiofile does not
like partial reads (Mantis 0004028).

Move code from the "mad" plugin.

Move code from the "mad" plugin.

.. instead of InputStream::LockRead(). The former is cancellable.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>

Error message was 'too few arguments for "find"' because the "base"
constraint was not supported, and no other constraints remained.

Fixes missing song length on high-latency Opus files.

According to tests with 320 kbit/s opus files with 60ms packets, we
need to skip up to 29 kB.

Turns out the lock-free code using atomics was not thread-safe.  The
given callback could be invoked by GLib before the source_id attribute
was assigned.  This commit changes the DeferredMonitor class to use a
Mutex to block the event loop until source_id is assigned.  This bug
does not exist in the 0.19 branch because it does not use the GLib
main loop anymore.

This corrects a major mistake from commit 724a59aa - there was one
small thing that commit was supposed to do, and it failed.
AV_TIME_BASE is not a seek flag.