add Image source imageSmoothing option

Add Image smoothing checkbox to example

Test imageSmoothing option

src/ol/source/Source.js

@@ -140,6 +140,13 @@ class Source extends BaseObject {
     return this.wrapX_;
   }
 
+  /**
+   * @return {Object|undefined} Context options.
+   */
+  getContextOptions() {
+    return undefined;
+  }
+
   /**
    * Refreshes the source. The source will be cleared, and data from the server will be reloaded.
    * @api