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/track/video_track.dart

@@ -1,17 +1,18 @@
 import 'package:flutter_webrtc/flutter_webrtc.dart' as rtc;
+import 'package:meta/meta.dart';
 
 import '../proto/livekit_models.pb.dart' as lk_models;
 import 'track.dart';
 
 /// A video track will notify when its mediaTrack has changed.
 class VideoTrack extends Track {
-  //
   rtc.MediaStream _mediaStream;
 
   VideoTrack(
     String name,
     rtc.MediaStreamTrack mediaTrack,
     this._mediaStream,
+    // this._client,
   ) : super(
           lk_models.TrackType.VIDEO,
           name,
@@ -22,6 +23,7 @@ class VideoTrack extends Track {
 
   /// internal use
   /// {@nodoc}
+  @internal
   void setMediaStream(rtc.MediaStream stream) {
     _mediaStream = stream;
     notifyListeners();