background: only load a single background per idle
This is a minimum effort to help avoid stalling the main loop by doing a single background per idle at a lower priority than the redraw of GTK.
Ideally, libgnome-desktop would provide an asynchronous loading mechanism for the GdkPixbuf but that will basically require a rewrite of it given all the state it manages synchronously.
This also allows the background to snapshot "nothing" until it's texture has been loaded.
Related: #3167 (closed)