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

examples/xyz-offset.js

@@ -0,0 +1,37 @@
+var map, layer;
+
+// if tiles are not available, hide images
+//OpenLayers.Util.onImageLoadError = function() {
+//    this.style.display = "none";
+//}
+
+// called on body load
+function init() {
+
+    var extent = new OpenLayers.Bounds(
+        -13758743.4295939,  5591455.28887228, -13531302.3472101 , 5757360.4178881
+    );
+
+    map = new OpenLayers.Map({
+        div: "map",
+        maxExtent: new OpenLayers.Bounds(
+            -128 * 156543.0339, -128 * 156543.0339,
+            128 * 156543.0339, 128 * 156543.0339
+        ),
+        restrictedExtent: extent,
+        maxResolution: 611.496226171875, // corresponds to level 8 in the cache
+        numZoomLevels: 6,
+        projection: new OpenLayers.Projection("EPSG:900913"),
+        units: "m",
+        layers: [
+            new OpenLayers.Layer.XYZ(
+                "ESRI",
+                "http://sampleserver1.arcgisonline.com/ArcGIS/rest/services/Portland/ESRI_LandBase_WebMercator/MapServer/tile/${z}/${y}/${x}",
+                {zoomOffset: 8} // since our map maxResolution differs from cache max resolution
+            )
+        ]
+    });
+
+    map.zoomToExtent(extent);
+
+}