This 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.
cache `element.getOffsetParent()` to optimize

cache `element.getOffsetParent()` to optimize code after nice suggestion
from @anutron
Merge remote-tracking branch 'origin/sortable-fixed-position'