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 if the window or offsetParent  is scrolled.

This PR adjusts that by using window and offsetParent scroll to adjust the position 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.