Fix: exclude text processors when text is disabled

This fixes a bug observed with low-latency streams containing one or
more text representations. The bug prevented the liveCatchUpPlaybackRate
from being correctly applied following a buffer stall, leading to
unconstrained drift from the user-specified target latency.

The bug manifested as follows:

1. a buffer stall event occurs, the 'playbackStalled' field on
   StreamController is set to 'true'
2. 'StreamController.startPlaybackCatchUp' is called, it uses
   'StreamController.getBufferLevel' to obtain the lowest current audio,
   video text or fragmented text buffer level
3. in streams with a text or fragmented text representation, the recent
   change to disable text by default resulted in an empty buffer being
   maintained for that representation _unless_ text had been explicitly
   enabled for the player
4. the empty buffer prevents 'StreamController.startPlaybackCatchUp'
   from resetting the 'playbackStalled' field of 'StreamController' to
   'false', further, it assumes a buffer stall is ongoing, and resets
   the 'newRate' to '1.0', preventing latency from being reduced

This change then checks whether text has been enabled and only adds the
stream processors for textual representations to those returned by the
'Stream.getProcessors' if it is.

Tested on Firefox 78.02 & Chrome 84.0.414.79 on Ubuntu 20.04.

Author: 03k64

src/streaming/Stream.js

@@ -714,6 +714,8 @@ function Stream(config) {
     }
 
     function getProcessors() {
+        const isTextEnabled = textController.isTextEnabled();
+
         let arr = [];
 
         let type,
@@ -723,7 +725,9 @@ function Stream(config) {
             streamProcessor = streamProcessors[i];
             type = streamProcessor.getType();
 
-            if (type === Constants.AUDIO || type === Constants.VIDEO || type === Constants.FRAGMENTED_TEXT || type === Constants.TEXT) {
+            if (type === Constants.AUDIO || type === Constants.VIDEO) {
+                arr.push(streamProcessor);
+            } else if ((type === Constants.FRAGMENTED_TEXT || type == Constants.TEXT) && isTextEnabled) {
                 arr.push(streamProcessor);
             }
         }