If we have repeats on for a full-sized image, then if the background
is displayed scaled (for example, in a desktop preview mode) then we
can get artifacts along the edge of the background where the repeat
of the opposite edge is blended in by bilinear scaling. So turn off
repeats when the screen and background image sizes match.
https://bugzilla.gnome.org/show_bug.cgi?id=634833