QGLWidget offscreen rendering and GL_MAX_TEXTURE_SIZE

Dear Qt-ers, I wrote a small application displaying an heightmap with a texture and I would like to export a snapshot bigger than my screen. I found the convenient QGLWidget::renderPixmap() but unfortunately it renders the scene with a “virtual” OpenGL1.1 device that has a limit of 1024×1024 for the texture size (GL_MAX_TEXTURE_SIZE). As expected, it works fine for small texture, but above the 1024² limit, the heightmap is not textured. Do I have alternative to achieve my “high-res” capture of the scene? Thanks a lot