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.

test_rendering/spec/ol/layer/vector.test.js

@@ -75,6 +75,9 @@ describe('ol.rendering.layer.Vector', function() {
 
     it('renders fill/stroke batches correctly with the canvas renderer', function(done) {
       map = createMap('canvas');
+      source = new ol.source.Vector({
+        overlaps: false
+      });
       addPolygon(100);
       addCircle(200);
       addPolygon(250);