fix: Fix support of getAllThumbnails when using shaka.dash.TimelineSegmentIndex (#7508)

Author: avelad

lib/dash/segment_template.js

@@ -1116,6 +1116,19 @@ shaka.dash.TimelineSegmentIndex = class extends shaka.media.SegmentIndex {
     return ref;
   }
 
+  /**
+   * @override
+   */
+  forEachTopLevelReference(fn) {
+    this.fitTimeline();
+    for (let i = 0; i < this.getNumReferences(); i++) {
+      const reference = this.get(i + this.numEvicted_);
+      if (reference) {
+        fn(reference);
+      }
+    }
+  }
+
   /**
    * Fill in a specific template with values to get the segment uris
    *