Changed elementOffset to position

When the container div has an offset, elementOffset don't match to the draggable position. Position does.

I haven't try to change elementOffset in _renderProxy since I've got no rendering problem. (maybe i'm wrong).

To test, try this for the container div 

#container {
                display: block;
                position: absolute;
                top: 50%;
                left: 50%;
                transform: translateY(-50%) translateX(-50%);
}