def draw_icon(point, icon)
      x, y = point.pixel(@bounding_box)
      icon.draw(@canvas, x, y)
    end
def draw_icon(point, icon)
      x, y = point.pixel(@bounding_box)
      icon.draw(@canvas, x, y)
    end
initialize the image with a (lat/lng) bounding box of the tile it
 represents
 draws passed icon at passed position