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