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.

Check {GCC,CLANG}_VERSION==0 or >0 instead of using defined(), which
may render undefined behavior.

GCC_VERSION>0 implies defined(__GNUC__).