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. |