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.

Replacing GLib macros.