The destructor will call this in any case.

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.

Replaces g_ascii_strup() and allows building the Vorbis encoder
without GLib.

Enables using range-based "for".

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

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

The MPD core will add this as a prefeix.

Replaces GLib's GError.

Improves quality by not squeezing 32 bit samples down to 16 bit, and
then back to 32 bit floating point.  Reduces CPU usage by skipping a
conversion step.