Limit image layer requests to layer extent

This has two nice consequences that can be seen in the wms-image.js and mapbuide-untiled.js examples:
 * no images are requested when you browse outside of the layer extent
 * when the layer extent is within the viewport extent, cached images are used since the intersecting extent is the same for multiple viewport extents

examples/mapguide-untiled.js

@@ -15,6 +15,7 @@ var bounds = [
 var map = new ol.Map({
   layers: [
     new ol.layer.Image({
+      extent: bounds,
       source: new ol.source.MapGuide({
         projection: 'EPSG:4326',
         url: agentUrl,
@@ -24,8 +25,7 @@ var map = new ol.Map({
           MAPDEFINITION: mdf,
           FORMAT: 'PNG'
         },
-        ratio: 2,
-        extent: bounds
+        ratio: 2
       })
     })
   ],

examples/wms-image.js

@@ -11,11 +11,11 @@ var layers = [
     source: new ol.source.MapQuest({layer: 'sat'})
   }),
   new ol.layer.Image({
+    extent: [-13884991, 2870341, -7455066, 6338219],
     source: new ol.source.ImageWMS({
       url: 'http://demo.opengeo.org/geoserver/wms',
       params: {'LAYERS': 'topp:states'},
-      serverType: 'geoserver',
-      extent: [-13884991, 2870341, -7455066, 6338219]
+      serverType: 'geoserver'
     })
   })
 ];