Disable Context Set from Undo when toggling between Render Results

Image shows it all. When you're comparing between images in the render results you create a LOT of undo steps, objectively losing your previous Undos for no actual gain in function.

Changing between slots doesn't need to be an undo state, in my opinion. I'm giving away a simple addon that fixes this for whoever finds it useful, but I do think it should be the default behavior.