background: Mipmap wallpaper when shrinking
So as to minimize jaggies for wallpaper that is >= double the resolution of the screen.
Closes: gnome-shell#254 (closed)
Note: This fixes aliasing in the main wallpaper (gnome-shell#254 (closed)), but doesn't fix aliasing in workspace thumbnails. I can't yet figure out how to solve the latter without them turning black (cogl bug?).