don't show old backbuffer tiles after loading. p=h0nIg, r=me (see #3376)

git-svn-id: http://svn.openlayers.org/trunk/openlayers@12116 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Tile/Image/IFrame.js

@@ -166,7 +166,6 @@ OpenLayers.Tile.Image.IFrame = {
         //bind a listener to the onload of the iframe so that we
         // can register when a tile has finished loading.
         var onload = function() {
-            this.show();
             //normally isLoading should always be true here but there are some
             // right funky conditions where loading and then reloading a tile
             // with the same url *really*fast*. this check prevents sending