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

The destructor will call this in any case.

Some "ar" implementations don't allow empty archives, and failed to
build it.  The real reason (which is hidden with GNU binutils) is that
our Makefile.am was incorrect, and didn't completely disable
libxiph.a.

See https://bugs.musicpd.org/view.php?id=4526

Rename struct Encoder to PreparedEncoder, and add a new (abstract)
class Encoder which represents one encoder instance.