Let small windows appear at full size in the overlay if there is room

Previously we forced all windows to shrink at least as much as the
workspace itself did. Now we allow to shrink by a smaller amount (or
not at all) if they'll still fit within their allotted slot.

http://bugzilla.gnome.org/show_bug.cgi?id=571192

js/ui/workspaces.js

@@ -431,7 +431,7 @@ Workspace.prototype = {
 
             let size = Math.max(clone.actor.width, clone.actor.height);
             let desiredSize = global.screen_width * fraction;
-            let scale = Math.min(desiredSize / size, 1.0);
+            let scale = Math.min(desiredSize / size, 1.0 / this.scale);
 
             Tweener.addTween(clone.actor, 
                              { x: xCenter - 0.5 * scale * clone.actor.width,