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

This reverts commit 995cd954.  It was
a bad idea because it caused assertion failures when a decoder plugin
aborted with an exception.

Prepare to reuse the same code for a new Vorbis decoder plugin based
on libvorbis instead of libvorbisfile.

Allow decoders to throw std::runtime_error.

Initialize the OggStreamState as soon as the first page is seen; no
special code needed in the caller.