unregister load and error handlers after blank image is loaded. r=sbrunner,pgiraud (closes #3419)

git-svn-id: http://svn.openlayers.org/trunk/openlayers@12286 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Tile/Image.js

@@ -328,7 +328,6 @@ OpenLayers.Tile.Image = OpenLayers.Class(OpenLayers.Tile.BackBufferable, {
      */
     createBackBuffer: function() {
         var frame = this.frame.cloneNode(false);
-        OpenLayers.Event.stopObservingElement(this.imgDiv);
         frame.appendChild(this.imgDiv);
         this.imgDiv = null;
         return frame;
@@ -340,6 +339,7 @@ OpenLayers.Tile.Image = OpenLayers.Class(OpenLayers.Tile.BackBufferable, {
      */
     onImageLoad: function() {
         var img = this.imgDiv;
+        OpenLayers.Event.stopObservingElement(img);
         img.style.display = "";
         this.isLoading = false;
         this.events.triggerEvent("loadend");