wayland: Improve checks when flushing scroll events

wayland: Improve checks when flushing scroll events

If we get gdk_wayland_seat_flush_frame_event() with no previous event to be
flushed, we fallback into the scroll event checks. However, there's no check
performed there as to whether it really scrolled, so it'd always send a smooth
scroll event with 0/0 deltas in this case.

This should be mostly harmless, but still, we should only end up emitting scroll
events if those really happened.