Replacing GLib macros.

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.