Fixed startup error in ocio.py.

Because it didn't import IECore, it was relying on another config file importing IECore before it ran. IECore.ConfigLoader makes no guarantees about load order within a particular directory of the searchpath, so for certain load orders errors ensued.
Corrected a bug where the data window of an image is displayed incorrectly in the ImageViewer.
Simplified some arithmetic.
Merge pull request #744 from goddardl/ImageViewerUiFix

Image Viewer UI Fix