RT has a straighten tool and perspective controls which allow correction of converging verticals, but as far as I know, there’s not a really slick way to do this common adjustment. Hence this suggestion for a new tool, rather like the straighten tool, except to draw 2 lines on the verticals, e.g. -
Apologies for the crap lines but I couldn’t quickly see how to do them properly in Gimp…
On releasing the mouse on the 2nd line, or perhaps pressing a button, RT would calculate and set the Rotation and Vertical perspective sliders, and leave the user to check or uncheck Auto-fill.
I think this would be a good feature, and it seemed when I searched that few apps can do this.