feature request: Warp Transformation constraints

[pierrelabreche]

The Warp transformation could include assistance for leveling horizontal and vertical lines ( such as horizon and vertical features )

The user would have the ability to define one or more leveling lines ( horizontal or vertical ) - and the Warp Transformation would allow adjusting corners of the warp rectangle along a path that maintains the leveling lines horizontal or vertical, and continually reposition the other corners to satisfy the leveling constraints.