Allows database plugins to use the EventThread, e.g. for CURL
integration.

Our local EventLoop instance was never run; it was not usable.

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.

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

Apparently, this makes old clang versions happy ("'alignas' attribute
cannot be applied to types).

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.

Some database plugins don't use a Storage (e.g. UPnP), and with this
plugin, DatabaseDetachSong() can crash.

Eliminate dependency on io_thread_get().

Previously, there was no special code to convert stereo to
multi-channel.  The generic solution for this was to convert to mono,
and then copy the result to all channels.  That's a pretty bad
solution, but at least something which always renders audio.  MPD does
something, instead of failing.

Now that MPD has proper support for multi-channel (by defining the
channel order), we can do better than that.  It is a (somewhat) common
case to play back stereo music on a DAC which can only do
multi-channel.  The best approach here is to copy the stereo channels
to front-left and front-right, and apply the "silence" pattern to all
other channels.

The unit test was wrong as well.  D'oh!

These were disabled by accident.

Move this sample rate fixup from the ALSA output plugin to PcmExport,
where it belongs.

DSD_U32 packs four bytes instead of one large "sample", thus the
sample rate is one quarter of the input sample rate.  This fixes a
rather critical DSD_U32 playback problem.

Prepare for DSD sample rate fixups.