wlr_seat_touch: Don't destroy touch point with surface

When the surface is destroyed clear it's reference but wait for the up event to destroy the touch point via wlr_seat_touch_notify_up().

If the surface is destroyed before the up event we end up with incomplete sequences sent to the client like

[915821.276] wl_touch@3.down(146, 2475027, wl_surface@38, 0, 236.000000, 515.000000)
[915821.608] wl_touch@3.frame()
[915821.637] wl_touch@3.motion(2475027, 0, 236.000000, 515.000000)
[915821.779] wl_touch@3.frame()

so there's never an up event. While it should be something like

[2461229.051] wl_touch@3.down(81, 3236959, wl_surface@34, 0, 218.000000, 478.000000)
[2461229.435] wl_touch@3.frame()
[2461229.484] wl_touch@3.motion(3236959, 0, 218.000000, 478.000000)
[2461229.636] wl_touch@3.frame()
[2461277.520] wl_touch@3.up(82, 3237007, 0)
[2461277.681] wl_touch@3.frame()

this confuses tookits intepreting the next down event incorrectly. So don't destroy the touch point too early.

Closes: #32 (closed)