Add imageRatio option for VectorImage layers

src/ol/layer/VectorImage.js

@@ -2,10 +2,13 @@
  * @module ol/layer/VectorImage
  */
 import BaseVectorLayer from './BaseVector.js';
+import {assign} from '../obj.js';
 import CanvasVectorImageLayerRenderer from '../renderer/canvas/VectorImageLayer.js';
 
 /**
  * @typedef {import("./BaseVector.js").Options} Options
+ * @property {number} [imageRatio=1] Ratio by which the rendered extent should be larger than the
+ * viewport extent A larger ratio avoids cut images during panning, but will cause a decrease in performance.
  */
 
 
@@ -23,7 +26,25 @@ class VectorImageLayer extends BaseVectorLayer {
    * @param {Options=} opt_options Options.
    */
   constructor(opt_options) {
+    const options = opt_options ? opt_options : /** @type {Options} */ ({});
+
+    const baseOptions = assign({}, options);
+    delete baseOptions.imageRatio;
     super(opt_options);
+
+    /**
+     * @type {number}
+     * @private
+     */
+    this.imageRatio_ = options.imageRatio !== undefined ? options.imageRatio : 1;
+
+  }
+
+  /**
+   * @return {number} Ratio between rendered extent size and viewport extent size.
+   */
+  getImageRatio() {
+    return this.imageRatio_;
   }
 
   /**