Dear Maintainer,

Unless, the desktop background image have exactly the right
dimensions, spanning the image across multiple monitors will
result in contant-color bars on the multi-monitor
background.

It seems that SPAN operates like SCALE, but it would be
better for SPAN to operate like ZOOM, so that it just works
as the user expects in most cases. An ideal solution would
be to replace the single SPAN case with SPAN-ZOOM and
SPAN-SCALE cases, so that one could use SCALE-like behavior
if one really wanted to.

The included patch changes SPAN's behavior so that it works
like ZOOM and not like SCALE.

This is essentially the same as the patch submitted in
#608411 but applies to libgnome-desktop-3-2 instead of
libgnome-desktop-2-17. A similar fix would be nice for
libgnome-desktop-3-4 (in experimental as of this writing).