gears.wallpaper: Make code more readable

by introducing some extra variables that server as some documentation.

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

lib/gears/wallpaper.lua.in

@@ -139,7 +139,9 @@ function wallpaper.maximized(surf, s, ignore_aspect, offset)
     if offset then
         cr:translate(offset.x, offset.y)
     elseif not ignore_aspect then
-        cr:translate(((geom.width / aspect_w) - w) / 2, ((geom.height / aspect_h) - h) / 2)
+        local scaled_width = geom.width / aspect_w
+        local scaled_height = geom.height / aspect_h
+        cr:translate((scaled_width - w) / 2, (scaled_height - h) / 2)
     end
 
     cr:set_source_surface(surf, 0, 0)