point.changeInAngle(dx, dy, ref)
Return the change in angle (in degrees) that is the result of moving the point from its previous position to its current position.
More specifically, this function computes the angle between the line from the
ref point to the previous position of this point (i.e. current position
-dy) and the line from the ref point to the current position of this point.
The function returns a positive value between 0 and 180 when the angle (in the direction from previous position of this point to its current position) is clockwise, and a negative value between 0 and -180 when the angle is counterclockwise. The function returns 0 if the previous and current positions of this point are the same (i.e. both