Alters the actor's relative position on the y-axis in the same way tile offsets could be altered in DukeRes. Also like DukeRes, the valid numbers that can be added/subtracted range from 0-128. Adding to this struct member will move the tile up, and subtracting from it moves it down. Since this is a member of the tsprite array, it is important to note that the new position of the tile number will NOT reflect the actual position of the actor. The altered position exists in appearance only.