Fix error when switching from 2 touches drag event to transform

Without my fix it was possible to generate a 2 touches drag event ( which is good ) and enter in to gesture.transform code and generate a transform event without taking care of finishing the drag event.
And because it was not handled it generated an error because the _first was not to true and making the _pos.startCenter = undefined

So I added code to handle correctly the switch from 2 touches drag to transform. It generate the dragend event and take care to set _first to true to handle the first event of transform correctly.