Fixes window width/height and centering on Xlib.

window_default() never actually set the display resolution (probably since it would have led to an infinite loop). I've modified this to actually work (and be a little more xlib friendly). So now, rooms larger than the default 640x480 will not be squished.

There is still the chance of a few stray loops (I caught one in window_set_set_size()), since it seems like a lot of the xlib code was borrowed from win32 (which is fundamentally different from xlib, obviously).