Use the existing PcmDither library.

Since opening the PcmVolume object can now fail, this case must be
handled.

Replaces Reset() and eliminates the AudioFormat parameters from the
Convert() method.

Wake up the OutputThread only if it hasn't already been woken up and
if it isn't already in the playback loop.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Fixes deprecation warnings.

Implements Mantis ticket 0003340.

False positives in gcc 4.7.

Let the caller deal with a failure.

The other audio_format attributes are not used.

Pass audio_output objects around instead of void pointers.  This will
give some more control to the plugin, and prepares for non-blocking
audio outputs.

The output thread could hang indefinitely after finishing CANCEL,
because it could have missed the signal while the output was not
unlocked in ao_command_finished().

This patch removes the wait() call after CANCEL, and adds the flag
"allow_play" instead.  While this flag is set, playback is skipped.
With this flag, there will not be any excess wait() call after the
pipe has been cleared.

This patch fixes a bug that causes mpd to discontinue playback after
seeking, due to the race condition described above.