These attributes are printed in the "outputs" response, and the new
command "outputset" allows setting new values.

No attributes are currently implemented.

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.

Moved code from AudioOutputControl::InternalOpen2().

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

Use the new HugeAllocator as backend for SliceBuffer and call
HugeDiscard() when the last chunk was returned.

This is harmful for memory usage, because it forces the kernel to
allocate physical memory, even before playback has started.  No bug
has been found in a few years with this, so it's safe to assume that
this code is not necessary.

Preparation for cross-fading support in the output thread.