How do I flip upside down fonts in FTGL

I just use FTGL to use it in my app.
I want to use the version FTBufferFont to render font but it renders in the wrong way.
The font(texture?buffer?) is flipped in the wrong axis.

I want to use this kind of orthographic settings:

    void enable2D(int w, int h)
        winWidth = w;
        winHeight = h;
        glViewport(0, 0, w, h);
        //I don't even want to swap the 3rd and 4th param
        //because I like to retain the top-left as the origin
        glOrtho(0, w, h, 0, 0, +1);

I render the font like this:

    //No pushing and popping of matrices

    //No translation

    font.Render("Hello World!", -1, position, spacing, FTGL::RenderMode::RENDER_FRONT);

On the other forums, they said, just scaled the y-axis down to -1, but it wont work in mine. Example:

font is above the screen

I can’t see relevant problem like in mine in google so I decide to ask this here again.

How can I invert texture’s y-axis without modifying its source code ? (assume its read only)