Adding tileloadstart event to layer.

For this to be useful (i.e. so listeners can see/modify the tile url), a change in Tile.Image is required so we do not fire the loadstart event before we have the url to load.

lib/OpenLayers/Layer/Grid.js

@@ -196,6 +196,9 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
      * element - {DOMElement} A reference to layer.events.element.
      *
      * Supported event types:
+     * tileloadstart - Triggered when a tile starts loading. Listeners receive
+     *     an object as first argument, which has a tile property that
+     *     references the tile that starts loading.
      * tileloaded - Triggered when each new tile is
      *     loaded, as a means of progress update to listeners.
      *     listeners can access 'numLoadingTiles' if they wish to keep
@@ -971,6 +974,7 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
             if (this.numLoadingTiles == 0) {
                 this.events.triggerEvent("loadstart");
             }
+            this.events.triggerEvent("tileloadstart", {tile: tile});
             this.numLoadingTiles++;
         };