Simplified tile queue; using the tile's beforedraw listener.

Since draw is the only tile operation that we defer, the tile queue can be an array of tiles and queue handling can be simplified. We now use the beforedraw event to defer drawing, and remove all occurrences of a tile from the tile queue when we draw it.

Instead of layers that want to defer tile drawing having to override the tile's draw method, layers can now abort drawing by returning false from a beforedraw listener, and later call draw(true) to draw the tile directly, without clearing it first.

lib/OpenLayers/Tile/Image.js

@@ -204,6 +204,7 @@ OpenLayers.Tile.Image = OpenLayers.Class(OpenLayers.Tile, {
      * it can be reused in a new location.
      */
     clear: function() {
+        OpenLayers.Tile.prototype.clear.apply(this, arguments);
         var img = this.imgDiv;
         if (img) {
             OpenLayers.Event.stopObservingElement(img);