Perspective skew on the transform cage

This lets the user skew the perspective of the rectangle with some arbitrary affine transform. The user can access this by dragging a corner of the transform cage while holding Ctrl.

The issue names no files or tests. Start by locating the transform cage's corner-drag interaction and the existing affine-transform path, then determine how Ctrl-drag is handled. Done means a Ctrl-dragged corner applies the intended perspective skew while ordinary cage transforms continue to work.

索引モデルが issue の本文から書いたものです。