move addTile code to the base class (OpenLayers.Layer.Grid). r=crschmidt (closes #2904)

git-svn-id: http://svn.openlayers.org/trunk/openlayers@11575 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Layer/Grid.js

@@ -629,9 +629,7 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
 
     /**
      * APIMethod: addTile
-     * Gives subclasses of Grid the opportunity to create an 
-     * OpenLayer.Tile of their choosing. The implementer should initialize 
-     * the new tile and take whatever steps necessary to display it.
+     * Create a tile, initialize it, and add it to the layer div. 
      *
      * Parameters
      * bounds - {<OpenLayers.Bounds>}
@@ -641,7 +639,8 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
      * {<OpenLayers.Tile>} The added OpenLayers.Tile
      */
     addTile:function(bounds, position) {
-        // Should be implemented by subclasses
+        return new OpenLayers.Tile.Image(this, position, bounds, null, 
+                                         this.tileSize, this.tileOptions);
     },
     
     /**