wayland: Find touch grab sequence in subsurfaces also
With libdecor, window moving/resizing only works with the pointer, not with touch. The meta_wayland_pointer_can_grab_surface checks for subsurfaces, but the meta_wayland_touch_find_grab_sequence does not.
Add a similar subsurface check to meta_wayland_touch_find_grab_sequence.
Closes: #2872 (closed)