Previously, there was no special code to convert stereo to
multi-channel.  The generic solution for this was to convert to mono,
and then copy the result to all channels.  That's a pretty bad
solution, but at least something which always renders audio.  MPD does
something, instead of failing.

Now that MPD has proper support for multi-channel (by defining the
channel order), we can do better than that.  It is a (somewhat) common
case to play back stereo music on a DAC which can only do
multi-channel.  The best approach here is to copy the stereo channels
to front-left and front-right, and apply the "silence" pattern to all
other channels.

Use some odd number that will expose problems with remaining samples
after optimized vector operations.

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.

Didn't compile on GLib < 2.16.