Allow extents to restrict tile ranges requested from the server

The addition of full extent tile ranges also allows us to simplify wrapX
handling for tile layers. By limiting wrapX to true and false as possible
values, we can remove a lot of guessing logic.

examples/wms-custom-tilegrid-512x256.js

@@ -15,7 +15,7 @@ for (var i = 0, ii = resolutions.length; i < ii; ++i) {
   resolutions[i] = startResolution / Math.pow(2, i);
 }
 var tileGrid = new ol.tilegrid.TileGrid({
-  origin: ol.extent.getBottomLeft(projExtent),
+  extent: [-13884991, 2870341, -7455066, 6338219],
   resolutions: resolutions,
   tileSize: [512, 256]
 });
@@ -25,7 +25,6 @@ var layers = [
     source: new ol.source.MapQuest({layer: 'sat'})
   }),
   new ol.layer.Tile({
-    extent: [-13884991, 2870341, -7455066, 6338219],
     source: new ol.source.TileWMS({
       url: 'http://demo.boundlessgeo.com/geoserver/wms',
       params: {'LAYERS': 'topp:states', 'TILED': true},

examples/wms-tiled-wrap-180.js

@@ -13,8 +13,7 @@ var layers = [
     source: new ol.source.TileWMS({
       url: 'http://demo.boundlessgeo.com/geoserver/ne/wms',
       params: {'LAYERS': 'ne:ne_10m_admin_0_countries', 'TILED': true},
-      serverType: 'geoserver',
-      wrapX: true
+      serverType: 'geoserver'
     })
   })
 ];