input: prevent cursor notifications from pointer and tablet tool

... at the same time. Omit cursor notifications from
a pointer when a tablet tool (stylus/pen) is in
proximity. We expect to get cursor notifications
from the tablet tool instead.

src/seat.c

@@ -553,6 +553,8 @@ seat_init(struct server *server)
 	}
 	wlr_cursor_attach_output_layout(seat->cursor, server->output_layout);
 
+	wl_list_init(&seat->tablet_tools);
+
 	input_handlers_init(seat);
 }