gears.wallpaper: Create smaller image surface (FS#1087)

When no wallpaper exists yet, instead of creating a black image surface which
covers all the screens, just create a surface for the screen which we need. This
means that way less pixels have to be uploaded to the X11 server, which should
be faster.

Signed-off-by: Uli Schlachter <psychon@znc.in>