Applying software volume to S16 samples means several bits of
precision are lost; at 25% volume, two bits are lost.  Additionally,
dithering adds some noise.

The problem gets worse when you apply the software volume code twice:
for the software mixer volume, and again for the replay gain.  This
loses some more precision and adds even more dithering noise, which
can become audible (see
https://github.com/MusicPlayerDaemon/MPD/issues/542).

By converting everything to 24 bit, we need to shift only two bits to
the right instead of ten, losing nearly no precision, and dithering is
not needed.  Even if the output device is unable to play S24 directly,
we can convert back to S16 with only one stage of dithering.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/542

Prepare for a new mode which may convert to a different sample format
when applying volume, to reduce dithering.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

While this assert() was useful when we had to track the object's state
manually, there was no practical purpose other than verifying old
code, and it complicates our new C++ code.

Instead of just adding a rectangular random value before shifting back
to the normal scale, use the existing PcmDither library.

Prepare for adding state.

Remove GLib dependency from some headers.

The other audio_format attributes are not used.

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

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

1024 is 2^10, and allows gcc to use bit shift operations instead of
multiplication / division.