LensDistortOp bug and compatibility with negative formats.

Fixed a bug in the LensDistortOp where it has been applying the distortion upside down.
Until now it has been unnoticeable as the only lens model in Cortex is symmetrical in the Y axis. However, it becomes apparent in external lens models such as IE's Anamorphic Model.
Added support to the LensDistortOp for images with an offset display window.
Added test cases that compare the output of the LensDistortOp to the LensDistort node in nuke.