Code simplification.

Be exception-safe.

Prepare full C++ exception support in the decoder thread.

Prepare full C++ exception support in the player thread.

The C++ compiler will auto-generate move and copy
constructors/operators for us.

Failure must be indicated by returning false.

Reduce header clutter.

Now that CloseFilter() may be called again, we can reduce code
duplication.

This allows calling CloseFilter() multiple times.

With the configuration "replay_gain_handler none", the Filter pointers
are never initialized, which can crash MPD.

 https://bugs.musicpd.org/view.php?id=4571

Prepare to add more modes.

The "packed" attribute triggers a clang 4.0 warning, and it's not
necessary.  All we want is correct allocation of this
dynamically-sized struct.