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.

release v0.19.19

This function exists since FFmpeg 3.1.  Fix a build failure with
FFmpeg 3.0.

Currently the switch statement is invalidated by ss.format being overwritten
with the default value of PA_SIMPLE_S16NE which results in white noise during
playback as my server is expected S16LE (S16NE).
Signed-off-by: Earnestly <zibeon@gmail.com>

These bug reports describe problems with some FFmpeg codecs:

 https://bugs.musicpd.org/view.php?id=4564
 https://bugs.musicpd.org/view.php?id=4568
 https://bugs.musicpd.org/view.php?id=4572

According to the FFmpeg bug tracker, a call to
avcodec_parameters_to_context() is required after
avcodec_alloc_context3():

 https://trac.ffmpeg.org/ticket/5781

This requirement was previously undocumented.