Don't create media state for text until ready to stream.

We start streaming text once subtitles have been enabled.
With external text, we used to create a media state for it
as soon as text was added even if captions were disabled
and we did not actually stream, although no text was
being parsed and shown.
This lead to the following problem:
If external text was added with subtitles disabled, we
created the media state with no actual cues in it.
Once the subtitles were enabled, we saw that a media
state existed for text and assumed it had content.
So, we started streaming, but nothing was shown.
This change makes us only create the media state if
subtitles are enabled and defer creation otherwise.

Closes #1938.

Change-Id: Iee39c30fbd6b7f0abe7772dfb75cba561fcc9998

lib/media/streaming_engine.js

@@ -575,9 +575,16 @@ shaka.media.StreamingEngine.prototype.loadNewTextStream = async function(
   await this.setupStreams_(streamSet);
   if (this.destroyed_) { return; }
 
+  const showText = this.playerInterface_
+      .mediaSourceEngine
+      .getTextDisplayer()
+      .isTextVisible();
+
+  const streamText = showText || this.config_.alwaysStreamText;
+
   if ((this.textStreamSequenceId_ == currentSequenceId) &&
       !this.mediaStates_.has(ContentType.TEXT) &&
-      !this.unloadingTextStream_) {
+      !this.unloadingTextStream_ && streamText) {
     const presentationTime = this.playerInterface_.getPresentationTime();
     const needPeriodIndex = this.findPeriodForTime_(presentationTime);