add setDisplacement method

src/ol/style/Image.js

@@ -203,6 +203,16 @@ class ImageStyle {
     return abstract();
   }
 
+  /**
+   * Set the displacement.
+   *
+   * @param {Array<number>} displacement Displacement.
+   * @api
+   */
+  setDisplacement(displacement) {
+    this.displacement_ = displacement;
+  }
+
   /**
    * Set the opacity.
    *