Update dragging offset based on _firstTrigger

This will ensure that the view associated with the trigger will be centered when the trigger occurs.

Basically what I noticed is that with a very small offset, the view will not come on screen should the library be using the default `kMCStop1` for controlling the offsets. This is most prevalent when working on landscape iPad table views, as the image will be inset by about 256 pixels in this case.

What I did was just use the `_firstTrigger` variable to control the offset, so that when you hit the offset, the view is completely visible.