wlr_seat_touch: Destroy the touchpoint on client destroy

Since e26217c5, touchpoints can outlive surfaces. This works fine as long as the client stays around, but fails horribly otherwise; therefore we have to make sure that touchpoints don't outlive their clients.

Fixes #35 (closed)

(cherry picked from commit 15dc71d603ad4b0393c5d8d045499dda78b1e42b)