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/internal/events.dart

@@ -1,11 +1,15 @@
 // added
 import 'package:flutter_webrtc/flutter_webrtc.dart' as rtc;
+import 'package:visibility_detector/visibility_detector.dart';
 import 'package:meta/meta.dart';
 
 import '../events.dart';
+import '../track/track.dart';
+
+abstract class InternalEvent implements LiveKitEvent {}
 
 @internal
-abstract class EngineIceStateUpdatedEvent with EngineEvent {
+abstract class EngineIceStateUpdatedEvent with EngineEvent, InternalEvent {
   final rtc.RTCIceConnectionState iceState;
   final bool isPrimary;
   const EngineIceStateUpdatedEvent({
@@ -35,3 +39,25 @@ class EnginePublisherIceStateUpdatedEvent extends EngineIceStateUpdatedEvent {
           isPrimary: isPrimary,
         );
 }
+
+@internal
+class TrackStreamUpdatedEvent with TrackEvent, InternalEvent {
+  final Track track;
+  final rtc.MediaStream stream;
+  const TrackStreamUpdatedEvent({
+    required this.track,
+    required this.stream,
+  });
+}
+
+@internal
+class TrackVisibilityUpdatedEvent with TrackEvent, InternalEvent {
+  final String rendererId;
+  final Track track;
+  final VisibilityInfo? info; // null means disposed
+  const TrackVisibilityUpdatedEvent({
+    required this.rendererId,
+    required this.track,
+    required this.info,
+  });
+}