Cherry-pick WriterAudio.GetSecondsRemaining

Now on completion of the voice over clip the number of seconds remaining is 0

Assets/Fungus/Docs/CHANGELOG.txt

@@ -9,6 +9,7 @@ Unreleased
  
 ## Changed
  - Block will LogError when an exception is caught from a Command being Executed.
+ - WriterAudio.GetSecondsRemaining reports 0 if not playing. Thanks to KVinS.
 
 v3.13.6
 ======

Assets/Fungus/Scripts/Components/WriterAudio.cs

@@ -63,7 +63,7 @@ namespace Fungus
         {
             if (IsPlayingVoiceOver)
             {
-                return targetAudioSource.clip.length - targetAudioSource.time;
+                return targetAudioSource.isPlaying ? targetAudioSource.clip.length - targetAudioSource.time : 0f;
             }
             else
             {