clutter: Update picked actor on scroll events

On one hand this avoids crashes early after startup if the very first pointer event is a scroll event, since the stage did not pick an actor for the pointer device yet.

On the other hand, scroll events have some likelihood to change the actor under the pointer even though it doesn't move. We still want to cross towards the new actor under the pointer ASAP, without waiting for later events.

Closes: #3112 (closed)