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

My concept with `class CancellableOperation` doesn't work properly,
because the kernel may continue to write to the given buffer as soon
as the read finishes.

To fix this, this commit adds `class ReadOperation` which owns the
buffer and the `struct iovec`.  Instances of this class persist until
the read really finishes, even if the operation is canceled.

And un-deprecate "pause" without parameter (toggles pause).  I have no
idea why it was deprecated long ago; the deprecation notice was copied
from the ancient MPD wiki.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/944

release v0.21.26

Skip the beginning of a sector if the last seek was odd, and clear the
buffer on seek.

This implements the feature that was missing/broken in this bug
report: https://github.com/MusicPlayerDaemon/MPD/issues/930

Prepare for an implementation without InputStream.

Similar to commit 4e2a551f but for
decoder plugins.  This is tailored for the FFmpeg decoder plugin which
implements some protocols (e.g. RTSP) as demuxer plugin.

Skip the beginning of a sector if the last seek was odd, and clear the
buffer on seek.

Same as e10b867f but it got lost in
the merge, because the v0.22 branch uses a different way to detect
supported protocols at runtime.

Support opus header gain tags and match opus playback volume to other tracks when ReplayGain is enabled.

Found with -Wdouble-promotion
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Closes https://github.com/MusicPlayerDaemon/MPD/issues/846

GCC5 cannot build mpd.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Eliminate some duplicate code.

This fixes a spurious "single" mode bug which occurs when using "play"
or "seek" to start playback on the song that is currently paused: in
that case, the main thread never queues the next song, and at the end
of the song, the player thread exits Run(), stopping playback, and
after that, the main thread starts the next song without considering
"single" mode.

By calling OnPlayerSync(), we ensure that the main thread gets a
chance to queue the next song before the player thread exits the Run()
loop.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/850