Added support for display windows with non-zero origins.
Added support for reading and writing images with non-zero origins.
GafferImage.ImageWriter is now able to write out black images of the default format rather than just throwing an exception.
Added test cases for the above.
Modified GafferImage.Reformat to add support for images with negative or offset formats.
Fixed an issue where the output display window would occasionaly be wrong by a pixel.
Fixed a bug