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/transport.dart

@@ -62,7 +62,7 @@ class PCTransport extends Disposable {
   }
 
   late final negotiate = Utils.createDebounceFunc(
-    () => createAndSendOffer(),
+    (void _) => createAndSendOffer(),
     cancelFunc: (f) => _cancelDebounce = f,
     wait: Timeouts.debounce,
   );