make default behaviour for an icon with no specified offset or calcuateOffset() function be to have a standard calculateOffset() function setting the offset point at the center of the icon

git-svn-id: http://svn.openlayers.org/trunk/openlayers@1553 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Icon.js

@@ -39,8 +39,12 @@ OpenLayers.Icon.prototype = {
     initialize: function(url, size, offset, calculateOffset) {
         this.url = url;
         this.size = (size) ? size : new OpenLayers.Size(20,20);
-        this.offset = (offset) ? offset : new OpenLayers.Pixel(0,0);
-        this.calculateOffset = calculateOffset;
+        this.offset = offset;
+        this.calculateOffset = (calculateOffset) ? calculateOffset
+                                 : function(size) {
+                                     return new OpenLayers.Pixel(-(size.w/2), 
+                                                                -(size.h/2));
+                                   };
 
         this.imageDiv = OpenLayers.Util.createAlphaImageDiv();
     },