start dragging given sprite

def start_drag(self, sprite, cursor_x = None, cursor_y = None):
        """start dragging given sprite"""
        cursor_x, cursor_y = cursor_x or sprite.x, cursor_y or sprite.y

        self._mouse_down_sprite = self._drag_sprite = sprite
        sprite.drag_x, sprite.drag_y = self._drag_sprite.x, self._drag_sprite.y
        self.__drag_start_x, self.__drag_start_y = cursor_x, cursor_y
        self.__drag_started = True