Set window size from actual window size, not gl context

This seems to resolve the problem that the opengl size is not the same as the window size - the window is created with the correct size by setup_window, but returns the size of the gl context, not the window.

By just ignoring the gl context size, everything seems to work, even if the window is large or small.

I don't know if this is the right way to do things, but raising it here for discussion anyway.