getResolution() is only to be called on a baselayer, so no need to ask the map to getExtent() -- we just ask ourselves

git-svn-id: http://svn.openlayers.org/branches/openlayers/2.0@1281 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Layer.js

@@ -278,7 +278,7 @@ OpenLayers.Layer.prototype = {
      */
     getResolution: function() {
         var viewSize = this.map.getSize();
-        var extent = this.map.getExtent();
+        var extent = this.getExtent();
         return Math.max( extent.getWidth()  / viewSize.w,
                          extent.getHeight() / viewSize.h );
     },