closest_point
Description:
public double closest_point (double x, double y, out double on_path_x, out double on_path_y, out int node_pos_before)
  
  Figure out what and where on a path is closest to arbitrary coordinates.
Returns the length along the path where the closest point was encountered.
Parameters:
| this | 
           a Path  | 
      
| x | 
           x coordinate.  | 
      
| y | 
           y coordinate  | 
      
| on_path_x | 
           return location for x coordinate on the path that was closest  | 
      
| on_path_y | 
           return location for y coordinate on the path that was closest  | 
      
| node_pos_before | 
           the node position interpreted before this position was deemed the closest coordinate.  |