Yes you're right, this due unfortunatly to an OpenGL bug i supposed.
Anyway, i found a way to pass that bug by first setting a working resolution for supermodel and OpenGl (say, 640x480 for example), launch supermodel and then, switch back to the desired resolution (320x240 in my case). I had to play with the size of the framebuffer a bit to get a clean and undistorted image

Now it's working like a charm on my old arcade CRT
