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 results of the LensDistort node in nuke to the Cortex LensDistortOp.

This pull request depends on the "ImageDiffOp" pull request in order for the tests to succeed.