crop property
Live crop rect, seeded from CropConfig.initialRect when a crop mode
activates and mutated by Stage's recognizer (1-pointer drag inside the
rect) or the Cropper widget's handles. Consumers read it (e.g. on
apply) via Stage.of(context).crop.value. Reset to Rect.zero when
no crop mode is active.
Implementation
final ValueNotifier<Rect> crop;