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.

externs/olx.js

@@ -4366,6 +4366,7 @@ olx.source.TileImageOptions.prototype.wrapX;
  *            cacheSize: (number|undefined),
  *            format: (ol.format.Feature|undefined),
  *            logo: (string|olx.LogoOptions|undefined),
+ *            overlaps: (boolean|undefined),
  *            projection: ol.ProjectionLike,
  *            state: (ol.source.State|undefined),
  *            tileClass: (function(new: ol.VectorTile, ol.TileCoord,
@@ -4415,6 +4416,17 @@ olx.source.VectorTileOptions.prototype.format;
 olx.source.VectorTileOptions.prototype.logo;
 
 
+/**
+ * This source may have overlapping geometries. Default is `true`. Setting this
+ * to `false` (e.g. for sources with polygons that represent adminstrative
+ * boundaries or TopoJSON sources) allows the renderer to optimise fill and
+ * stroke operations.
+ * @type {boolean|undefined}
+ * @api
+ */
+olx.source.VectorTileOptions.prototype.overlaps;
+
+
 /**
  * Projection.
  * @type {ol.ProjectionLike}
@@ -5797,6 +5809,7 @@ olx.source.TileWMSOptions.prototype.wrapX;
  *     format: (ol.format.Feature|undefined),
  *     loader: (ol.FeatureLoader|undefined),
  *     logo: (string|olx.LogoOptions|undefined),
+ *     overlaps: (boolean|undefined),
  *     strategy: (ol.LoadingStrategy|undefined),
  *     url: (string|ol.FeatureUrlFunction|undefined),
  *     useSpatialIndex: (boolean|undefined),
@@ -5849,6 +5862,17 @@ olx.source.VectorOptions.prototype.loader;
 olx.source.VectorOptions.prototype.logo;
 
 
+/**
+ * This source may have overlapping geometries. Default is `true`. Setting this
+ * to `false` (e.g. for sources with polygons that represent adminstrative
+ * boundaries or TopoJSON sources) allows the renderer to optimise fill and
+ * stroke operations.
+ * @type {boolean|undefined}
+ * @api
+ */
+olx.source.VectorOptions.prototype.overlaps;
+
+
 /**
  * The loading strategy to use. By default an {@link ol.loadingstrategy.all}
  * strategy is used, a one-off strategy which loads all features at once.