void OnMouseDown()
    {
        screenPoint = Camera.main.WorldToScreenPoint(transform.position);

        offset = transform.position - Camera.main.ScreenToWorldPoint(
            new Vector3(Input.mousePosition.x, transform.position.y, transform.position.z));
    }
    void OnMouseDrag()
    {
        Vector3 curScreenPoint = new Vector3(Input.mousePosition.x, transform.position.y, transform.position.z);

        Vector3 curPosition = Camera.main.ScreenToWorldPoint(curScreenPoint) + offset;
        if (draggable)
            transform.position = curPosition;
    }