Preparing for https://github.com/MusicPlayerDaemon/MPD/issues/42

Time to move on, two years after 2017.

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

Closes https://github.com/MusicPlayerDaemon/MPD/issues/607

This exception is usually thrown by class DecoderBridge, but the Opus
plugin (ab)uses it as well, so we need to catch it.

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

Similar to commit 32380d1d, these are
the final parts for really fixing
https://github.com/MusicPlayerDaemon/MPD/issues/469

These options are exclusive.

First part of the "real" fix for
https://github.com/MusicPlayerDaemon/MPD/issues/469

This can be used for other purposes as well.

Eliminates a number of allocations, because callers don't need to copy
the strings to a newly allocated buffer only to null-terminate them.
And most callers don't need to have a null-terminated string.

Let the caller do this.  Our GetSuffix() function was broken anyway.

Prepare for templated functions.

Just like std::condition_variable, which however has no way to specify
the std::mutex directly.