2:1 button in sharpen previews

[tomczak]

Would it make sense to make the 2:1 button a toggle instead of momentary? I often want to see the enlarged preview as I adjust settings - it works, but I need to keep changing the zoom often.

p.s. I think I discovered a way to keep the 2:1 depressed - not sure if intended but it works: press the 2:1 button and slide the mouse (still pressed) to the area outside of the button.

[jsachs]

Unintentional, but that does seem to work.