moving cursor in sissor mode: keep a stack for previous seeds in sequence shared between GUI and algorithm. The top element is the current seed. GUI keep sending current cursor coordinates to algorithm, and algorithm keeps computing the path, send it back to GUI, where GUI visuializes it.
Enter : finish the current contour
Ctrl + Enter: close current contour and fish
Ctrl + '+' : zoom in
Ctrl + '-' : zoom out
Arrow key : navigate
Backspace: when scissoring, delete the last seed; otherwise, delete selected contour. Select a contour by moving onto it. Selected contour is red, un-selected ones are green.