Adding a zoomOffset property to XYZ and TMS layers. This allows you to have a map with a maximum resolution that differs from the maximum cache resolution (if you want to show a subset of OSM for example). r=ahocevar (closes #2616)

git-svn-id: http://svn.openlayers.org/trunk/openlayers@10650 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Layer/XYZ.js

@@ -31,6 +31,18 @@ OpenLayers.Layer.XYZ = OpenLayers.Class(OpenLayers.Layer.Grid, {
      */
     sphericalMercator: false,
 
+    /**
+     * APIProperty: zoomOffset
+     * {Number} If your cache has more zoom levels than you want to provide
+     *     access to with this layer, supply a zoomOffset.  This zoom offset
+     *     is added to the current map zoom level to determine the level
+     *     for a requested tile.  For example, if you supply a zoomOffset
+     *     of 3, when the map is at the zoom 0, tiles will be requested from
+     *     level 3 of your cache.  Default is 0 (assumes cache level and map
+     *     zoom are equivalent).
+     */
+    zoomOffset: 0,
+    
     /**
      * Constructor: OpenLayers.Layer.XYZ
      *
@@ -101,7 +113,7 @@ OpenLayers.Layer.XYZ = OpenLayers.Class(OpenLayers.Layer.Grid, {
             / (res * this.tileSize.w));
         var y = Math.round((this.maxExtent.top - bounds.top) 
             / (res * this.tileSize.h));
-        var z = this.map.getZoom();
+        var z = this.map.getZoom() + this.zoomOffset;
 
         var url = this.url;
         var s = '' + x + y + z;