Retain final key pitch on release.

Prior to the introduction of Zones when a touch was released the final key pitch was retained. The Zones logic in 1.0a1 would send "touch off" messages with the key value based on the last touch sample. Since the last touch sample (t1) is post release the x position is always 0 which resulted in the final key pitch always being the lowest key in the zone. Sounds with a long release would pitch downward to the root key of the zone on release (effected both Aalto and Kyma in my testing).

The patch changes the "touch off" key to match that of the last know touch (t2) prior to release.