If simple-touch ran on a compositor with multiple seats, and the first
one happened to have the touch capability while the second one didn't,
the handler for seat capabilities would destroy the wl_touch device it
created on the first call for the first seat when it was called a again
for the second seat that has not touch capabilities.
Fix this problem by creating a separate struct for each seat.
https://bugs.freedesktop.org/show_bug.cgi?id=78365
c3f03f557b