Resizing is done when fg_os{,_sdl}.cxx says so, not in every single frame.

(This defeats the purpose of the resize callback and makes "listening" to
the xsize useless.)

src/Main/renderer.cxx

@@ -336,10 +336,6 @@ FGRenderer::update( bool refresh_camera_settings ) {
     current__view->set_dirty();
 
     if ( refresh_camera_settings ) {
-        // update view port
-        resize( fgGetInt("/sim/startup/xsize"),
-                fgGetInt("/sim/startup/ysize") );
-
         // Tell GL we are switching to model view parameters
         glMatrixMode(GL_MODELVIEW);
         glLoadIdentity();