add getTile() function to Grid.js. with tests

git-svn-id: http://svn.openlayers.org/trunk/openlayers@981 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Layer/Grid.js

@@ -101,6 +101,30 @@ OpenLayers.Layer.Grid.prototype =
             this.tileSize = size.clone();
         }
     },
+    
+    /** 
+     * @param {String} tileID
+     * 
+     * @returns The OpenLayers.Tile with the corresponding ID from the grid.
+     *           if no tile is found, returns null
+     * @type OpenLayers.Tile
+     */
+    getTile: function(tileID) {
+        var foundTile = null;
+        if (this.grid) {
+
+            for(var iRow = 0; iRow < this.grid.length; iRow++) {
+                var row = this.grid[iRow];
+                for(var iCol = 0; iCol < row.length; iCol++) {
+                    var tile = row[iCol];
+                    if (tile.id == tileID) {
+                        foundTile = tile;
+                    }
+                }
+            }
+        }
+        return foundTile;
+    },
 
     /** This function is called whenever the map is moved. All the moving
      * of actual 'tiles' is done by the map, but moveTo's role is to accept