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>

lib/gears/wallpaper.lua.in

@@ -37,8 +37,9 @@ local function prepare_wallpaper(s)
     local img = surface(root.wallpaper())
 
     if not img then
-        -- No wallpaper yet
-        img = cairo.ImageSurface(cairo.Format.RGB24, root_geom.width, root_geom.height)
+        -- No wallpaper yet, create an image surface for just the part we need
+        img = cairo.ImageSurface(cairo.Format.RGB24, geom.width, geom.height)
+        img:set_device_offset(-geom.x, -geom.y)
     end
 
     local cr = cairo.Context(img)