scenes/opengl: Fix clipping with transformed screens
Software-based clipping is naive. It maps the clip region from the global screen coordinates and intersects it with window quads. If the window is transformed, the quads won't be clipped as expected.
Unfortunately, the OpenGL scene enables software-based geometry clipping if the screen transformed. It's not clear why it does so, perhaps as a performance optimization? Either way, this produces incorrect results when the screen is scaled.