Stop update images when livestream is paused

Fixing: when a livestream is paused, we force the playhead back in
if the playhead falls out of the seek range, which makes the paused
image change.
Now we'll jump when the user hits play.

Closes #982.

Change-Id: I3f9e05e15c710488477b70ea7642f007c771ea92

lib/media/playhead.js

@@ -340,8 +340,13 @@ shaka.media.Playhead.prototype.onSeekingToStartTime_ = function() {
  * @private
  */
 shaka.media.Playhead.prototype.onPollGapJump_ = function() {
-  if (this.video_.readyState == 0 || this.video_.seeking)
-    return;
+  // Don't gap jump before the video is ready to play.
+  if (this.video_.readyState == 0) return;
+  // Don't gap jump while seeking, to prevent a race condition.
+  if (this.video_.seeking) return;
+  // Don't gap jump while paused, so that you don't constantly jump ahead while
+  // paused on a livestream.
+  if (this.video_.paused) return;
 
   // When the ready state changes, we have moved on, so we should fire the large
   // gap event if we see one.