... not when we do an update.
We only repick when we handle events, not when we update. Perhaps
this is a mistake.
Since update runs before handle_event, this means that when we
drop a grab, update will notice the NULL surface, since we haven't
repicked after the event, and then we'll repick the correct surface.
The end result is that you see a root cursor after a grab ends,
rather than the correct window cursor.
This doesn't fix it, since the current surface becomes NULL when
we start the grab. But it does make the code here more correct when
we fix that bug.
de7a644656