Push the new mode to the filter instead of accessing global variables
through replay_gain_get_real_mode().

Let the function decoder_replay_gain() update
decoder_control::replay_gain_db instead of letting each decoder plugin
take care for that.

The "bool" type is built-in.

Improved support for scanning http:// resources.

Make sure the file header gets written at the beginning, before
_write() gets called.

Make sure the queue "owns" all song objects, so nobody else can free
them.

Eliminates explicit global initialisation.

GLib 2.16 was released more than 4 years ago.  Let's remove some cruft
from the glib_compat.h header, and avoid new cruft to it.

This plugin is horrible code, I mean it.  Last year, I tried hard to
fix it, but I figured would take less time to do a full rewrite.
Given that I don't even have any device that supports RAOP, I can't do
that properly.  After 16 months, nobody volunteered for fixing it.
Hereby, I delete it, because having no RAOP plugin is better than
having this mess.  Sorry.

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

Delete the now-unused pcm_byteswap library, and optimize the
pcm_export_state object.

Eliminate support for reverse endian samples from the MPD core.  This
moves a lot of complexity to the plugins that really need it (only
ALSA and CDIO currently).

Some compilers are very picky, but we really aren't interested in the
return value.