When using the stretch controls, bug had image 4X smaller than it should be.

With D28935, when using the custom stretch sliders, the image displayed while using the sliders
was 4X smaller than it should be (just the preview images--once the sliders were released, the image was
rendered fine). To test, load an image, make a custom stretch change (either changing shadows, midtones or highlights)
and the image displayed while changing the sliders should not change in size as it would without this fix.