Eliminate a use of GetGlobalConfig().

This fixes an old bug which caused the "unused" warnings to be
unreliable; only the first block in the list was marked as being
"used", no matter if it was really used, and the rest was never marked
as "used", suppressing all warnings for them.

AudioOutputControl::WaitForCommand() doesn't need this check.

This requires re-adding the reference to struct DecoderControl, which
was removed recently by commit
9f14e7a9

Make all uses of MusicChunk safe.

We can implement this now that we have a MusicBuffer reference.

This tiny amount of overhead allows omitting the MusicBuffer in
Clear().

Fixes shutdown memory leak.

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

Prepare to move some configuration settings from AudioOutput to
AudioOutputControl.

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

Will move attributes from struct AudioOutput that are specific to the
OutputThread.  The new struct AudioOutputControl is a holder for the
AudioOutput pointer.

This prepares for making the output list more dynamic, to allow moving
outputs to between partitions.

Reduce overhead by eliminating MultipleOutputs::ResetReopen().

Eliminate a dependency from MultipleOutputs on PlayerControl.