Video optimizations (#18)

* use events for `VideoTrackRenderer`

* logger

* implemented visibility detection

* optimize code with 1 param debounce func

* stop pending visibility event when disposed

* clean up

* mark internal

* report to track

* call dispose on pub and track

* use internal events to receive visibility events

* clean up visibility for disposed track

* listen to track visibility events on pub

* re-design

* quickly re-enable

* clean up

* clean up

lib/src/participant/remote_participant.dart

@@ -18,7 +18,6 @@ import 'participant.dart';
 /// Represents other participant in the [Room].
 class RemoteParticipant extends Participant {
   final SignalClient _client;
-
   SignalClient get client => _client;
 
   RemoteParticipant(
@@ -92,8 +91,6 @@ class RemoteParticipant extends Participant {
     final Track track;
     if (pub.kind == lk_models.TrackType.AUDIO) {
       // audio track
-      // await AudioManager().incrementSubscriptionCounter();
-
       final audioTrack = AudioTrack(pub.name, mediaTrack, stream);
       await audioTrack.start();
       track = audioTrack;
@@ -162,7 +159,7 @@ class RemoteParticipant extends Participant {
     if (pub is! RemoteTrackPublication) {
       // no publication exists for trackSid
       // or publication is not RemoteTrackPublication
-      // logger.warning('pub is not RemoteTrackPublication');
+
       await pub?.dispose();
       return;
     }