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

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Eliminate a use of GetGlobalConfig().

Fix breakage from commit bea5681f

Yet another C-style vtable replaced with C++.

Hide struct FilteredAudioOutput from the plugins, preparing for hiding
MPD's core internals.

Avoid accessing FilteredAudioFormat attributes directly.

Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

Circumvent filter_new() with a fake ConfigBlock.

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

The AudioOutput struct (which is exposed to all plugins) should not be
aware that it's being controlled by another thread.