give Map the concept of a base layer

git-svn-id: http://svn.openlayers.org/trunk/openlayers@641 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Map.js

@@ -63,6 +63,9 @@ OpenLayers.Map.prototype = {
     // OpenLayers.Pixel
     mouseDragStart: null,
 
+    /** @type OpenLayers.Layer */
+    baseLayer: null,
+
     /**
     * @param {DOMElement} div
     */    
@@ -147,6 +150,10 @@ OpenLayers.Map.prototype = {
             this.layerContainerDiv.appendChild(layer.div);
         }
         this.layers.push(layer);
+
+        if (layer.isBaseLayer()) {
+            this.baseLayer = layer;
+        }
         this.events.triggerEvent("addlayer");
     },