Changing extent structure back to single array: [minX, minY, maxX, maxY]

This means we'll have to have a new structure and new methods for 3D envelopes.

examples/wms-tiled.js

@@ -14,7 +14,7 @@ var layers = [
     source: new ol.source.TileWMS({
       url: 'http://demo.opengeo.org/geoserver/wms',
       params: {'LAYERS': 'topp:states', 'TILED': true},
-      extent: [[-13884991, 2870341], [-7455066, 6338219]]
+      extent: [-13884991, 2870341, -7455066, 6338219]
     })
   })
 ];