Changed the internal representation of Images to have an origin in the bottom left.

This rather large pull request changes the internal representation of Images to have their origin in the bottom left corner. This should hopefully make things more intuitive for the users whilst also removing the need to remap the values of plugs on an image node which refer to image space coordinates or transformations from the UI representation of the image space which has an origin in the bottom left to the internal representation where the origin is in the top left.

In addition to this, support has now been added for negative or offset display windows which addresses an existing issue.