Fix shaka-project#3783 captions not working after a period transition…

… on live DASH streams (shaka-project#3801)

Embedded CEA-608 captions don't work on multi period live DASH DAI streams after new periods appear in the manifest because commas are appended the streams originalId string, which disrupts some stream matching code in text_engine.js

The problem has been resolved by preventing makeTextStreamsForClosedCaptions() from altering the PeriodCombiner.textStreams_ array.

Fixes shaka-project#3783

lib/util/periods.js

@@ -80,7 +80,10 @@ shaka.util.PeriodCombiner = class {
 
   /** @return {!Array.<shaka.extern.Stream>} */
   getTextStreams() {
-    return this.textStreams_;
+    // Return a copy of the array because makeTextStreamsForClosedCaptions
+    // may make changes to the contents of the array. Those changes should not
+    // propagate back to the PeriodCombiner.
+    return this.textStreams_.slice();
   }
 
   /** @return {!Array.<shaka.extern.Stream>} */