give map::getResolution a fallback from baseLayer to the 1st layer if in allOverlays mode. r=tschaub (closes #2479)

git-svn-id: http://svn.openlayers.org/trunk/openlayers@10043 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Map.js

@@ -1926,6 +1926,11 @@ OpenLayers.Map = OpenLayers.Class({
         var resolution = null;
         if (this.baseLayer != null) {
             resolution = this.baseLayer.getResolution();
+        } else if(this.allOverlays === true && this.layers.length > 0) {
+            // while adding the 1st layer to the map in allOverlays mode,
+            // this.baseLayer is not set yet when we need the resolution
+            // for calculateInRange.
+            resolution = this.layers[0].getResolution();
         }
         return resolution;
     },