fixes #1242, for position fixed sortables

When the sortables offsetParent has `position = fixed`, by dragging a
sortable the cloned element being dragged has the wrong position.

This PR adjusts that by using window and offsetParent scroll to adjust
the value.

The buggy behaviour can be seen [in this jsFiddle](http://jsfiddle.net/B5yn5/) and in the [new spec](http://sergiofrilans.se/temp/MoreSpecs/mootools-ui-runner/index.php/more/Drag/Sortables_(position_absolute)) included in this PR.