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.