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.

Don't deinterleave manually, don't allocate memory.

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.

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.

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

Add the encoder_plugin method end().  This is important for the
recorder plugin.

Don't reset the ogg_stream_state object, because this discards the
end-of-stream packet that was just added.

For simplicity, the MPD core should not have to deal with packing.  It
is rarely used, and those plugins that need it should use the
pcm_export library instead.

Convert to padded 24 bit samples, instead of falling back to 16 bit.