Add 'strength' slider to lens distortion module
The lens distortion module is either on or off. For artistic reasons, it is often desirable to control the effect of a distortion. For example, removing all curves from a fisheye image using 'rectilinear' can create extreme stretching in the corners of an image. If the effect could be controlled, removing e.g. 50% of the fisheye bending would be possible. This slider should be located under the 'mode' widget and go from 0 to 100 or from 0 to 1. Using values over 1 could be allowed for 'over' correction/distortion.