Image compare : switching image B should not reset the preview area

[pierrelabreche]

When comparing different renderings produced by processing branches, I use the compare transformation, and select areas of interest for viewing

When the B image is changed, I would like PWP to keep the area being displayed in the preview port.

So, when a new image B is selected, the area being displayed for image A would not change, and the B side of the preview would be updated with the B image at the same offsets corresponding to image A.

[jsachs]

Implemented for next release.