using blank image to avoid the "Loading Image" frames on Firefox that were discussed during the review (see #3419)

git-svn-id: http://svn.openlayers.org/trunk/openlayers@12243 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Tile/Image.js

@@ -40,6 +40,15 @@ OpenLayers.Tile.Image = OpenLayers.Class(OpenLayers.Tile.BackBufferable, {
     frame: null, 
 
     /**
+     * Property: blankImageUrl
+     * {String} Does not result in a blank image on all browsers, but this is
+     * only required in some browsers as transparent backgroundImage, or as
+     * temporary image src while the tile is invisible and loading, so we don't
+     * care.
+     */
+    blankImageUrl: "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAQAIBRAA7",
+
+     /** 
      * Property: imageReloadAttempts
      * {Integer} Attempts to load the image.
      */
@@ -274,10 +283,9 @@ OpenLayers.Tile.Image = OpenLayers.Class(OpenLayers.Tile.BackBufferable, {
      * url - {String} or undefined to hide the image
      */
     setImgSrc: function(url) {
-        this.imgDiv.style.display = "none";
+        var img = this.imgDiv;
-        if (url) {
+        img.style.display = "none";
-            this.imgDiv.src = url;
+        img.src = url ? url : this.blankImageUrl;
-        }
     },
     
     /**

lib/OpenLayers/Tile/Image/IFrame.js

@@ -28,15 +28,7 @@ OpenLayers.Tile.Image.IFrame = {
      */ 
     useIFrame: null,
 
-    /**
+   /**
-     * Property: blankImageUrl
-     * {String} This is only used as background image for the eventPane, so we
-     * don't care that this doesn't actually result in a blank image on all
-     * browsers
-     */
-    blankImageUrl: "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAQAIBRAA7",
-
-    /**
     * Method: updateBackBuffer
     * Update the <backBufferData>, and return a new or reposition the
     * backBuffer. When a backbuffer is returned, the tile's markup is not