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/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