Update dragging offset based on _firstTrigger
Update dragging offset based on _firstTrigger

This will ensure that the view associated with the trigger will be centered when the trigger occurs.

luosam1123/MCSwipeTableViewCell,andaluz/MCSwipeTableViewCell,andreamazz/MCSwipeTableViewCell,jaohoang/MCSwipeTableViewCell,alikaragoz/MCSwipeTableViewCell,domenicosolazzo/MCSwipeTableViewCell,hoowang/MCSwipeTableViewCell,luosam1123/MCSwipeTableViewCell,yangchenghu/MCSwipeTableViewCell,zoonooz/MCSwipeTableViewCell,AlexanderMazaletskiy/MCSwipeTableViewCell,domenicosolazzo/MCSwipeTableViewCell,SuPair/MCSwipeTableViewCell,andreamazz/MCSwipeTableViewCell,AlexanderMazaletskiy/MCSwipeTableViewCell,ewerx/MCSwipeTableViewCell,adhoclabs/MCSwipeTableViewCell