effects: Disable blur and contrast effect when screen is locked

The results of the blur and the background contrast effect will never be seen if the screen locker is active.

In order to avoid wasting computational resources, we could temporarily disable the blur effect until the screen is unlocked.

Furthermore, given that the blur effect shrinks the clip region, it is guaranteed that the compositor will have to render the desktop window behind the lockscreen window. With this change, all windows behind the screen locker will be properly clipped away.