Layer and tile API update.

The tile now has responsibility for resolving feature ids and fetching feature data given x, y pixel offsets with getFeatureId and getFeatureData methods.  The layer has corresponding getFeatureId and getFeatureData methods that take a map location, lookup the appropriate tile, and delegate to the tile for the rest of the work.

lib/OpenLayers/Tile/UTFGrid.js

@@ -32,6 +32,15 @@ OpenLayers.Tile.UTFGrid = OpenLayers.Class(OpenLayers.Tile, {
      */
     url: null,
     
+    /**
+     * Property: utfgridResolution
+     * {Number}
+     * Ratio of the pixel width to the width of a UTFGrid data point.  If an 
+     *     entry in the grid represents a 2x2 block of pixels, the 
+     *     utfgridResolution would be 2.  Default is 4.
+     */
+    utfgridResolution: 4,
+    
     /** 
      * Property: json
      * {Object}
@@ -135,6 +144,68 @@ OpenLayers.Tile.UTFGrid = OpenLayers.Class(OpenLayers.Tile, {
         }
         this.isLoading = false;
     },
+    
+    /**
+     * Method: getFeatureData
+     * Get feature data associated with a pixel offset.
+     *
+     * Parameters:
+     * i - {Number} X-axis pixel offset (from top left of tile)
+     * j - {Number} Y-axis pixel offset (from top left of tile)
+     *
+     * Returns:
+     * {Object} The UTFGrid data corresponding to the given pixel offset.
+     */
+    getFeatureData: function(i, j) {
+        var data;
+        if (this.json) {
+            data = this.json.data[this.getFeatureId(i, j)];
+        }
+        return data;
+    },
+    
+    /**
+     * Method: getFeatureId
+     * Get the identifier for the feature associated with a pixel offset.
+     *
+     * Parameters:
+     * i - {Number} X-axis pixel offset (from top left of tile)
+     * j - {Number} Y-axis pixel offset (from top left of tile)
+     *
+     * Returns:
+     * {Object} The feature identifier corresponding to the given pixel offset.
+     */
+    getFeatureId: function(i, j) {
+        var id;
+        if (this.json) {
+            var resolution = this.utfgridResolution;
+            var code = this.resolveCode(this.json.grid[ 
+                    Math.floor((j) / resolution) 
+                ].charCodeAt(
+                    Math.floor((i) / resolution)
+                ));
+            id = this.json.keys[code];
+        }
+        return id;
+    },
+
+    /**
+     * Method: resolveCode
+     * Resolve the UTF-8 encoding stored in grids to simple number values.  
+     *     See the UTFGrid spec for details.
+     *
+     * Parameters:
+     * key - {Integer}
+     *
+     * Returns:
+     * {Integer} Adjusted key for non-escaped chars
+     */
+    resolveCode: function(key) {
+        if (key >= 93) key--;
+        if (key >= 35) key--;
+        key -= 32;
+        return key;
+    },
 
     /**
      * Method: parseData