see if the polygon has been clicked on.
        Consider it clicked if the pixel is within 6 of the point

def clicked(self, px, py):
        '''see if the polygon has been clicked on.
        Consider it clicked if the pixel is within 6 of the point
        '''
        if self.hidden:
            return None
        for i in range(len(self._pix_points)):
            if self._pix_points[i] is None:
                continue
            (pixx,pixy) = self._pix_points[i]
            if abs(px - pixx) < 6 and abs(py - pixy) < 6:
                self._selected_vertex = i
                return math.sqrt((px - pixx)**2 + (py - pixy)**2)
        return None