Introduce new overlaps option for Vector and VectorTile sources

Instead of deciding whether to batch fills and strokes by looking at the
opacity of the style, we now rely on user input.

examples/mapbox-vector-tiles-advanced.js

@@ -50,6 +50,7 @@ var map = new ol.Map({
           '© <a href="http://www.openstreetmap.org/copyright">' +
           'OpenStreetMap contributors</a>',
         format: new ol.format.MVT(),
+        overlaps: false,
         tileGrid: new ol.tilegrid.TileGrid({
           extent: ol.proj.get('EPSG:3857').getExtent(),
           resolutions: resolutions

examples/mapbox-vector-tiles.js

@@ -21,6 +21,7 @@ var map = new ol.Map({
           '© <a href="http://www.openstreetmap.org/copyright">' +
           'OpenStreetMap contributors</a>',
         format: new ol.format.MVT(),
+        overlaps: false,
         tileGrid: ol.tilegrid.createXYZ({maxZoom: 22}),
         tilePixelRatio: 16,
         url: 'http://{a-d}.tiles.mapbox.com/v4/mapbox.mapbox-streets-v6/' +

examples/osm-vector-tiles.js

@@ -92,6 +92,7 @@ var map = new ol.Map({
     new ol.layer.VectorTile({
       source: new ol.source.VectorTile({
         format: format,
+        overlaps: false,
         tileGrid: tileGrid,
         url: 'http://{a-c}.tile.openstreetmap.us/' +
             'vectiles-land-usages/{z}/{x}/{y}.topojson'

examples/topojson.js

@@ -29,7 +29,8 @@ var style = new ol.style.Style({
 var vector = new ol.layer.Vector({
   source: new ol.source.Vector({
     url: 'data/topojson/world-110m.json',
-    format: new ol.format.TopoJSON()
+    format: new ol.format.TopoJSON(),
+    overlaps: false
   }),
   style: function(feature) {
     // don't want to render the full world polygon, which repeats all countries