SeatInterface currently has a separation of kwin's focus scope to
pointer input with early return guards in notifyPointerEnter and
notifyPointerLeave where clients don't get pointer events.
However we don't update the initial state when a drag is started, this
patch notifies sends a pointer leave to the new drag target before the
data_device enter so things are consistent.
This also brings it in line with Weston and Mutter.
notifyPointerLeave has it's early return removed as for wayland windows
as we know nothing will have pointer focus.
