Apparently, libmpcdec sets gain/peak variables to zero if they are not
present.  This clashes with our formula and results in bogus values
which cause noise during playback.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/640

Add a `constexpr` constructor and several `constexpr` methods to
construct a DecoderPlugin at compile time, in a way which allows
adding new methods later without having to edit each plugin.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

https://bugs.musicpd.org/view.php?id=4656 describes a crash due to
division by zero because frame.samples==0.  This should never happen,
but apparently can happen after seeking.  The best we can do is to
just ignore this frame.

Declare two named elements.  An enum should not be used as an array
index, as this is error prone.

Replaces decoder_data() and others.

Replaces decoder_initialized().

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

Eliminates some duplicate code, and as a side effect, this works
around clang 3.8 compiler warning because a negative value was
shifted.

This was lost in commit 8ead8f7e, because this was never implemented
for the new API.

Allow the compiler to invalidate buffer contents.

Remove the "whence" parameter that is not actually necessary, and only
complicates the InputStream implementations.