New crossOriginKeyword property.

Set to 'anonymous' for Layer.OSM and Layer.Bing, and not used by default for other layers.

lib/OpenLayers/Tile/Image.js

@@ -87,6 +87,16 @@ OpenLayers.Tile.Image = OpenLayers.Class(OpenLayers.Tile, {
      * the tile image.
      */
     canvasContext: null,
+    
+    /**
+     * APIProperty: crossOriginKeyword
+     * The value of the crossorigin keyword to use when loading images. This is
+     * only relevant when using <getCanvasContext> for tiles from remote
+     * origins and should be set to either 'anonymous' or 'use-credentials'
+     * for servers that send Access-Control-Allow-Origin headers with their
+     * tiles.
+     */
+    crossOriginKeyword: null,
 
     /** TBD 3.0 - reorder the parameters to the init function to remove 
      *             URL. the getUrl() function on the layer gets called on 
@@ -305,7 +315,7 @@ OpenLayers.Tile.Image = OpenLayers.Class(OpenLayers.Tile, {
             } else {
                 OpenLayers.Event.observe(img, "load", load);
                 OpenLayers.Event.observe(img, "error", load);
-                img.crossOrigin = null;
+                delete img.crossOrigin;
                 img.src = this.blankImageUrl;
             }
         }
@@ -324,7 +334,9 @@ OpenLayers.Tile.Image = OpenLayers.Class(OpenLayers.Tile, {
         img.style.opacity = 0;
         if (url) {
             // don't set crossOrigin if the url is a data URL
-            img.crossOrigin = url.substr(0, 5) === 'data:' ? null : '';
+            if (this.crossOriginKeyword && url.substr(0, 5 !== 'data:')) {
+                img.crossOrigin = this.crossOriginKeyword;
+            }
             img.src = url;
         }
     },