it was uninitialized before

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

Not initialising granulepos leads to it having arbitrary values in the
encoded stream including possibly negative values which are not valid
and confuse opusdec.  Explicitly initialise opus_encoder::granulepos
to avoid that problem.

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

.. and move the plugins to src/encoder/plugins/.

Replacing GLib macros.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

I recently opened a bug: http://bugs.musicpd.org/view.php?id=3787

The main problem is that opus encoder config for signal overwrote
bitrate setting.

The "opusinfo" program complained about preskip value that was too
small.  This commit uses OPUS_GET_LOOKAHEAD to obtain the number of
frames that shall be silence at the beginning.

If there are multiple pages, the last partial page must be flushed.