warp and crop tool : suggestion for hard stops at boundary

[pierrelabreche]

When cropping or warping image, it would be nice to disallow the output image borders to cross outside of the original image borders. This feature would create hard stops when dragging an edge, a point or adjusting the rotation angle outside of the image boundaries.
A toggle button or a other visible control in the transform dialog box could turn on and off this behavior.

Thus a coarse user gesture could bring the crop rectangle or warp quadrilateral all the way to the input image border.