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

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

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

Replaces GLib's GError.

This fixes a buffer corruption bug; pcm_buffer is not designed to be a
persistent buffers, and will discard anything between two consecutive
calls.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

That's an optional method.