gfxFloat new_x = m0 * *dx  +  m2 * *dy; gfxFloat new_y = m1 * *dx  +  m3 * *dy; *dx = new_x; *dy = new_y;
cairo_matrix_transform_distance(&mat, dx, dy);
    void TransformDistance(gfxFloat *dx, gfxFloat *dy) const {        gfxFloat new_x = m0 * *dx  +  m2 * *dy;        gfxFloat new_y = m1 * *dx  +  m3 * *dy;        *dx = new_x;        *dy = new_y;    }