Allow simultaneous navigation to be turned off in overlay comparison

[#1]

Currently, you can turn off simultaneous navigation for side by side comparison in cases (for example) where the printer returns a sample with additional space around the image and printer marks. When you go to overlay, however, the two versions are back to simultaneous navigation and despite the icon changing colour when you click on it, can't really be disabled.

It would be great to be able to use simultaneous navigation to line up the images, then 'flick' between them comparing them.

[#2]

You could also allow this with the pixel-by-pixel auto-compare feature as well - Mark a pixel on each version as the starting point, rather than assuming it's 0,0 on both versions.