Replaced both player_stop_decoder() invocations with player_dc_stop(),
which also cleans up the pipe.

In the "CANCEL" command handler, the decoder is stopped twice: first
by player_dc_stop(), then by dc_stop().  Remove the latter.

After a seek, wait until enough new chunks are decoded before starting
playback.  If this takes too long, send silence chunks to the audio
outputs meanwhile.

When playback is unpaused, pass the audio_format to
audio_output_all_open().  Don't assume that output_all.c remembers the
previous audio format.  Also check if there has been an audio format
yet.

When seeking into a new song, and the decoder for the new song fails
to start up, MPD forgot to send the "command_finished" signal to the
main thread.

When pc.next_song is reset due to a decoder failure, also reset the
player.queued flag.  player.queued must not be true when there is no
pc.next_song.

Reset player.xfade and player.buffering from within
player_seek_decoder(), not in the player_process_command() switch
statement.

The music_buffer is a global variable, and must not be freed until the
player thread exits.

Instead of passing individual buffers to audio_output_all_play(), pass
music_chunk objects.  Append all those chunks asynchronously to a
music_pipe instance.  All output threads may then read chunks from
this pipe.  This reduces MPD's internal latency by an order of
magnitude.

When a PAUSE command is received while the decoder starts, don't open
the audio device when the decoder becomes ready.  It's pointless,
because MPD will close if after that.

Moved some more cruft out of do_play().

Moved some cruft out of do_play().

Preparation for the next patch: since the output devices stay open
even when the player thread stops playing, we will need a persistent
music buffer.

In !NDEBUG, remember which audio_format is stored in every chunk and
every pipe.  Check the audio_format of every new data block appended
to the music_chunk, and the format of every new chunk appended to the
music_pipe.

Turn the music_pipe into a simple music_chunk queue.  The music_chunk
allocation code is moved to music_buffer, and is now managed with a
linked list instead of a ring buffer.  Two separate music_pipe objects
are used by the decoder for the "current" and the "next" song, which
greatly simplifies the cross-fading code.

Don't abort MPD when a sample format is not supported by pcm_volume().

The warning message "problems opening audio device while playing ..."
does not help at all, and should be removed.  At this point, the real
error message has already been logged by the output thread.

When a file is not seekable, MPD dropped the audio buffers before even
attempting to seek.  This caused noticable sound corruption.  Fix:
first attempt to seek, and only if that succeeds, call
audio_output_all_cancel().

Minimize header dependencies, again.

Renamed functions and variables.

Moved code which deals with all audio outputs at once into a separate
library.

The crossfading code shouldn't depend on the audio output code.  Pass
the current audio format to cross_fade_calc() and let it compare
directly, instead of using isCurrentAudioFormat().

Assertions on pc.command and pc.next_song.

When we reset pc.next_song if there is no song queued, this might
cause a race condition: the next song to be played is cleared, while
pc.command was already set.  Clear the "next_song" only if there is a
song queued for the current do_play() invocation.

After a player command (successful or not), reset pc.next_song,
because the queue is supposed to be empty then.  Otherwise,
playlist.queued and pc.next_song may disagree, which triggers an
assertion failure.

When the output device fails to play a chunk, set pc.error to
PLAYER_ERROR_AUDIO.  This way, the playlist knows that it should not
queue the next song.

Start the decoder thread when the player thread starts.  The decoder
thread is already stopped by the player thread.

player_control.thread contains the handle of the player thread, or
NULL if the player thread isn't running.

decoder_control.thread contains the handle of the decoder thread, or
NULL if the decoder thread isn't running.

Break from the loop instead of returning the function.  This calls
player_stop_decoder(), which in turn emits the PLAYLIST event.  This
allows the playlist to re-start the player.

The function player_seek_decoder() starts the decoder, but does not
check the return value of player_wait_for_decoder().

Make player_wait_for_decoder() and play_chunk() return a bool instead
of 0/-1.

The player_thread loop requests the next song from the playlist as
soon as the decoder finishes the song which is currently being played.
This is superfluous, and can lead to synchronization errors and wrong
results.  The playlist already knows when the player starts playing
the next song (player_wait_for_decoder() triggers the PLAYLIST event),
and will then trigger the scheduler to provide the next song.

player_wait_for_decoder() emits the PLAYLIST event on success, remove
the duplicate PLAYLIST event in its caller.

The "TAG" event is emitted by the player thread when the current
song's tag has changed.  Split this event from "PLAYLIST" and make it
a separate callback, which is more efficient.

Moved the software volume code to a separate library.