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.

src/ol/source/vector.js

@@ -94,6 +94,12 @@ ol.source.Vector = function(opt_options) {
    */
   this.format_ = options.format;
 
+  /**
+   * @private
+   * @type {boolean}
+   */
+  this.overlaps_ = options.overlaps == undefined ? true : options.overlaps;
+
   /**
    * @private
    * @type {string|ol.FeatureUrlFunction|undefined}
@@ -695,6 +701,14 @@ ol.source.Vector.prototype.getFormat = function() {
 };
 
 
+/**
+ * @return {boolean} The source can have overlapping geometries.
+ */
+ol.source.Vector.prototype.getOverlaps = function() {
+  return this.overlaps_;
+};
+
+
 /**
  * Get the url associated with this source.
  *