Only request tiles within the layer extent

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

examples/wmts.js

@@ -35,6 +35,7 @@ var map = new ol.Map({
     }),
     new ol.layer.Tile({
       opacity: 0.7,
+      extent: projectionExtent,
       source: new ol.source.WMTS({
         attributions: [attribution],
         url: 'http://services.arcgisonline.com/arcgis/rest/' +
@@ -48,7 +49,6 @@ var map = new ol.Map({
           resolutions: resolutions,
           matrixIds: matrixIds
         }),
-        extent: projectionExtent,
         style: 'default'
       })
     })