wayland: Fix touch event delivery to subsurfaces
Use SurfaceInterface::inputSurfaceAt when deciding which surface are we entering when sending a pointer event from a touch, in case it falls onto a subsurface.
Addresses https://gitlab.gnome.org/GNOME/gtk/-/issues/3737 on our end. Fixes teams/plasma-mobile/issues#173 (closed)
Edited by Aleix Pol Gonzalez