Fix missing method declaration

ol/renderer/vector was using the abstract ReplayGroup type while
referencing replayGroup.addDeclutter(), which was only defined
on the canvas and webgl implementations and was also defined
differently.

src/ol/render/ReplayGroup.js

@@ -18,6 +18,13 @@ class ReplayGroup {
    * @return {boolean} Is empty.
    */
   isEmpty() {}
+
+  /**
+   * @abstract
+   * @param {boolean} group Group with previous replay
+   * @return {Array<*>} The resulting instruction group
+   */
+  addDeclutter(group) {}
 }
 
 export default ReplayGroup;

src/ol/render/canvas/ReplayGroup.js

@@ -120,8 +120,7 @@ class CanvasReplayGroup extends ReplayGroup {
   }
 
   /**
-   * @param {boolean} group Group with previous replay.
-   * @return {import("../canvas.js").DeclutterGroup} Declutter instruction group.
+   * @inheritDoc
    */
   addDeclutter(group) {
     let declutter = null;

src/ol/render/webgl/ReplayGroup.js

@@ -67,10 +67,11 @@ class WebGLReplayGroup extends ReplayGroup {
   }
 
   /**
-   * @param {import("../../style/Style.js").default} style Style.
-   * @param {boolean} group Group with previous replay.
+   * @inheritDoc
    */
-  addDeclutter(style, group) {}
+  addDeclutter(group) {
+    return [];
+  }
 
   /**
    * @param {import("../../webgl/Context.js").default} context WebGL context.