fix memory leaks in glDrawElements (fixes #19)
Added dummy glXCreateWindow/glXDestroyWindow stubs

Just like glXCreateGLXPixmap/glXDestroyGLXPixmap they don't do
anything useful (and hence are not really correct). Still they are
needed for compiling supertuxkart 0.8 and avoid linking problems:

    undefined reference to `glXCreateWindow'
    undefined reference to `glXDestroyWindow'