Update JitsiStreamPresenterEffect.js (#7362)

* Update JitsiStreamPresenterEffect.js
Create/terminate the Web Worker on effect start/stop so that we don't leak them.

react/features/stream-effects/presenter/JitsiStreamPresenterEffect.js

 
         // Bind event handler so it is only bound once for every instance.
         this._onVideoFrameTimer = this._onVideoFrameTimer.bind(this);
-        this._videoFrameTimerWorker = new Worker(timerWorkerScript, { name: 'Presenter effect worker' });
-        this._videoFrameTimerWorker.onmessage = this._onVideoFrameTimer;
     }
 
     /**
         this._desktopElement.srcObject = desktopStream;
         this._canvas.width = parseInt(width, 10);
         this._canvas.height = parseInt(height, 10);
+        this._videoFrameTimerWorker = new Worker(timerWorkerScript, { name: 'Presenter effect worker' });
+        this._videoFrameTimerWorker.onmessage = this._onVideoFrameTimer;
         this._videoFrameTimerWorker.postMessage({
             id: SET_INTERVAL,
             timeMs: 1000 / this._frameRate
         this._videoFrameTimerWorker.postMessage({
             id: CLEAR_INTERVAL
         });
+        this._videoFrameTimerWorker.terminate();
     }
 
 }