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).

The other audio_format attributes are not used.

The ReplayGain filter clamped the gain to max. 100 % even if the
algorithm determined the signal needed a boost. That would result in any
such tracks being played with too low volume, effectively defeating the
purpose of the filter.

Fixes a clang warning.

Add an option for each audio output which enables the use of the
hardware mixer, instead of the software volume code.

This is hardware specific, and assumes linear volume control.  This is
not the case for hardware mixers which were tested, making this patch
somewhat useless, but we will use it to experiment with the settings,
to find a good solution.

Apply the replay gain in the output thread.  This means a new setting
will be active instantly, without going through the whole music pipe.
And we might have different replay gain settings for each audio output
device.

Make the audio_format argument non-const.  Allow the open() method to
modify it, to indicate that it wants a different input audio format
than the one specified.  Check that condition in chain_filter_open(),
and fail.

The pcm_volume library supports 32 bit samples, there's no reason to
disallow it in the filter plugin.

Detect misconfiguration during MPD startup, not when playback begins.

Indicate success and error.

Don't close child filters in the filter() method.

Wrap the AudioCompress library in a filter plugin.

This patch prepares support for floating point samples (and probably
other formats).  It changes the meaning of the "bits" attribute from a
bit count to a symbolic value.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

Don't abort the whole MPD process when the conversion fails.  This has
been a denial-of-service attack vector for years.

The volume plugin does not work for reverse_endian samples.

This mixer plugin may be used instead of the traditional global
software mixer.  It integrates with the "volume" filter plugin, and
can control the software volume of an audio output which has no
hardware mixer.

Added public methods to get and set the current volume.

The "volume" filter plugin will replace the current software volume
code.  One "volume" filter may be attached to each output device.
This will allow the user to use hardware mixers for some devices, and
software mixers for other devices at the same time.

Currently, neither the filter API nor the "volume" plugin is
integrated into MPD.

The filter API allows us to implement software volume as a pluggable
filter, and we will be able to integrate libraries like SoX.