fix: Fix captions not working after a period transition on live DASH …

…streams (#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 #3783

Change-Id: I0b99df9cf081a5ad340ac16344f5f0240633d683

lib/util/periods.js

@@ -75,7 +75,10 @@ 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();
   }
 
   /**