Alternatively manage features in an ol.Collection

ol.layer.Vector can now manage both an RTree and a Collection of features.
The new useSpatialIndex option allows to opt out of RTree management, and
the new ol.Collection type of the features option allows to opt in for
Collection management.

externs/olx.js

@@ -4977,12 +4977,13 @@ olx.source.TileWMSOptions.prototype.wrapX;
 
 /**
  * @typedef {{attributions: (Array.<ol.Attribution>|undefined),
- *     features: (Array.<ol.Feature>|undefined),
+ *     features: (Array.<ol.Feature>|ol.Collection.<ol.Feature>|undefined),
  *     format: (ol.format.Feature|undefined),
  *     loader: (ol.FeatureLoader|undefined),
  *     logo: (string|olx.LogoOptions|undefined),
  *     strategy: (ol.LoadingStrategy|undefined),
  *     url: (string|undefined),
+ *     useSpatialIndex: (boolean|undefined),
  *     wrapX: (boolean|undefined)}}
  * @api
  */
@@ -4998,8 +4999,9 @@ olx.source.VectorOptions.prototype.attributions;
 
 
 /**
- * Features.
- * @type {Array.<ol.Feature>|undefined}
+ * Features. If provided as {@link ol.Collection}, the features in the source
+ * and the collection will stay in sync.
+ * @type {Array.<ol.Feature>|ol.Collection.<ol.Feature>|undefined}
  * @api stable
  */
 olx.source.VectorOptions.prototype.features;
@@ -5052,6 +5054,29 @@ olx.source.VectorOptions.prototype.strategy;
 olx.source.VectorOptions.prototype.url;
 
 
+/**
+ * By default, an RTree is used as spatial index. When features are removed and
+ * added frequently, and the total number of features is low, setting this to
+ * `false` may improve performance.
+ *
+ * Note that
+ * {@link ol.source.Vector#getFeaturesInExtent},
+ * {@link ol.source.Vector#getClosestFeatureToCoordinate} and
+ * {@link ol.source.Vector#getExtent} cannot be used when `useSpatialIndex` is
+ * set to `false`, and {@link ol.source.Vector#forEachFeatureInExtent} will loop
+ * through all features.
+ *
+ * When set to `false`, the features will be maintained in an
+ * {@link ol.Collection}, which can be retrieved through
+ * {@link ol.source.Vector#getFeaturesCollection}.
+ *
+ * The default is `true`.
+ * @type {boolean|undefined}
+ * @api
+ */
+olx.source.VectorOptions.prototype.useSpatialIndex;
+
+
 /**
  * Wrap the world horizontally. Default is `true`. For vector editing across the
  * -180° and 180° meridians to work properly, this should be set to `false`. The