Only request tiles within the layer extent

Layer renderers are now responsible for requesting data within a limited extent.

examples/wms-tiled.js

@@ -10,10 +10,10 @@ var layers = [
     source: new ol.source.MapQuest({layer: 'sat'})
   }),
   new ol.layer.Tile({
+    extent: [-13884991, 2870341, -7455066, 6338219],
     source: new ol.source.TileWMS(/** @type {olx.source.TileWMSOptions} */ ({
       url: 'http://demo.opengeo.org/geoserver/wms',
       params: {'LAYERS': 'topp:states', 'TILED': true},
-      extent: [-13884991, 2870341, -7455066, 6338219],
       serverType: 'geoserver'
     }))
   })