gpoints is the function for drawing points,
gsortline is the function for drawing lines (lines are drawn by points clicking and connected by smoothing), and
DrawGUI is a demo.

- mouse left click to add a point
- mouse left click on a point and hold to
- movemouse right click on a point to remove
- keyboard input "s" to save the drawing
- keyboard input "r" to reset the drawing to previous saving
keyboard input "Delete" to delete the drawing