Replaces GLib's GError.

Avoid duplicating the tag.

Hide the definition from C code, to prepare the transition to C++.

replay_gain_get_real_mode() accesses dangerous globals.  Let's just
use the global variable replay_gain_mode, and assume "AUTO" means
"TRACK".  At this point, this is the best we can do, because neither
the decoder nor the player should know about the current replay gain
mode.  This is applied as a filter in the output thread.

Let the function decoder_replay_gain() update
decoder_control::replay_gain_db instead of letting each decoder plugin
take care for that.

Eliminate support for reverse endian samples from the MPD core.  This
moves a lot of complexity to the plugins that really need it (only
ALSA and CDIO currently).

This fixes seeking in the vorbis decoder during MPD startup.

Fixes assertion failure.

This checks both conditions: pending and running.  Fixes yet another
assertion failure!

.. and add a few code comments.

Don't read song.start_ms and song.end_ms, let the player thread manage
this logic instead.

Fixes one more assertion failure.

Fixes possible assertion failure.

When playing a CUE track, the player thread waited for the decoder to
become ready, and then sent a SEEK command to the beginning of the CUE
track.  If that is near the start of the song file, and the track is
short enough, the decoder could have finished decoding already at that
point, and seeking fails.

This commit makes this initial seek more robust: instead of letting
the player thread deal with the difficult timings, let the decoder API
emulate a SEEK command, and return it to the decoder plugin, as soon
as the plugin finishes its initialization.

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.

This was useless, because input_stream_read() blocks until data is
available.  Calling it in a loop doesn't make sense.

This is not necessary since all relevant input plugins have been moved
to the I/O thread, and there is no remaining useful buffer()
implementation.  This also fixes a busy loop when playing radio.