wayland: Fix sending wl_pointer.leave event to Xwayland during dnd

When dragging something from an X client to a Wayland client, we need to avoid sending a wl_pointer.leave event so Xwayland can keep updating the position of the dnd window, which contains a dnd icon.

Whether a pointer leave event needs to be sent should be determined when the dnd operation is started. If the current target surface has data devices associated with it, Xwayland should still have pointer focus.