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.

Tremor decoder is unusable since commit 2ee43c40. Sound is distorted to
the point where it's nothing but noise.

The data from vorbis_synthesis_pcmout() needs to be scaled and
clipped for 16 bit sample size. For reference see
http://lists.xiph.org/pipermail/tremor/2010-April/001642.html and
http://lists.xiph.org/pipermail/vorbis/2006-October/026513.html.
Signed-off-by: Marcin Jurkowski <marcin1j@gmail.com>

Replaces decoder_data() and others.

Replaces decoder_initialized().

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

Use libvorbis instead of libvorbisfile, which gives us more control
over the decoding process.

Move code from the "vorbis" decoder.

The old struct config_param remains only for top-level string options.

Reduce header dependencies.

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

Allows more compiler optimizations.