fix(text): Fix timestamp offset of CEA-608 cues

When we attach closed-caption cues to the text engine, we apply the
video timestamp offset to the cues, so that they can align with the
start of the stream. However, we previously forgot to apply that
offset to any nested cues.
This did not matter beforehand, since we previously ignored the start
and end times of nested cues. However, recent changes to the UI text
displayer changed that.
So this fixes nested cues to apply video timestamp offset.

Closes shaka-project#3782

Change-Id: I4c9140fcfa9bf94579f8d847e546ee4e2ec5eff4

lib/media/media_source_engine.js

@@ -48,8 +48,7 @@ shaka.media.MediaSourceEngine = class {
    * @param {!function(!Array.<shaka.extern.ID3Metadata>, number, ?number)=}
    *    onMetadata
    */
-  constructor(video, closedCaptionParser, textDisplayer,
-      onMetadata) {
+  constructor(video, closedCaptionParser, textDisplayer, onMetadata) {
     /** @private {HTMLMediaElement} */
     this.video_ = video;
 

lib/text/text_engine.js

@@ -345,6 +345,19 @@ shaka.text.TextEngine = class {
     return cues;
   }
 
+  /**
+   * @param {!shaka.text.Cue} cue the cue to apply the timestamp to recursively
+   * @param {number} videoTimestampOffset the timestamp offset of the video
+   * @private
+   */
+  applyVideoTimestampOffsetRecursive_(cue, videoTimestampOffset) {
+    cue.startTime += videoTimestampOffset;
+    cue.endTime += videoTimestampOffset;
+    for (const nested of cue.nestedCues) {
+      this.applyVideoTimestampOffsetRecursive_(nested, videoTimestampOffset);
+    }
+  }
+
   /**
    * Store the closed captions in the text engine, and append the cues to the
    * text displayer.  This is a side-channel used for embedded text only.
@@ -373,8 +386,7 @@ shaka.text.TextEngine = class {
 
       // Adjust CEA captions with respect to the timestamp offset of the video
       // stream in which they were embedded.
-      cue.startTime += videoTimestampOffset;
-      cue.endTime += videoTimestampOffset;
+      this.applyVideoTimestampOffsetRecursive_(cue, videoTimestampOffset);
 
       const keepThisCue =
           cue.startTime >= this.appendWindowStart_ &&