What about this small change?

while (1)
{
lvl_sky.v -= 0.002 * time_step;
lvl_sky.u += 0.002 * time_step;
wait(-0.1);
}

http://www.puppenheim.org/
http://www.wuerfels-welt.de
Old Game Tutorial: http://www.puppenheim.org/MITutorial.zip
Free Game Editor: http://puppenheim.org/AlienalsWeihnachtsmann.zip
http://www.puppenheim.org/blender_starting_main_settings.txt