Support upscale/downscale to original image size for preview

[slovak194]

Hi!

Thanks for a cool app!

I just played with the frog. If the original resolution is used - the "view original" switch is nice to use to visually inspect the quality by quickly switching back and forth. However, when a smaller (or different) output size is used (64 for example) it is hard to do as image jumps. It would be nice to have a switch that gives a preview of the same size as the original even if output resolution is altered.

Best regards, Alex