ui/src/main/java/ucar/nc2/ui/geoloc/Navigation.java
public Point2D worldToScreen(ProjectionPointImpl w, Point2D p) {
    p.setLocation( pix_per_world*w.getX() + pix_x0,
                   -pix_per_world*w.getY() + pix_y0);
    return p;
  }
public Point2D worldToScreen(ProjectionPointImpl w, Point2D p) {
    p.setLocation( pix_per_world*w.getX() + pix_x0,
                   -pix_per_world*w.getY() + pix_y0);
    return p;
  }
convert a world coordinate to a display point