Incorrect FPS value when WebcamPanel used with non-blocking mode

Execute code below and observe FPS value available in left bottom corner - it shows ~1000 instead correct 25 frames.

``` java
Webcam webcam = Webcam.getDefault();
webcam.setViewSize(WebcamResolution.VGA.getSize());
webcam.open(true);

WebcamPanel panel = new WebcamPanel(webcam, false);
panel.setPreferredSize(webcam.getViewSize());
panel.setFPSDisplayed(true);
```
