Calculate aspect based on window size, not image size

If we request a size of (z*w)×(z*h), and an aspect of w/h, (w/h)*(z*h) ≠
(z*w) is possible in double precision arithmetics. (I actually had this
case for two images only differing by 1px in width.) The WM would then
ignore our resize request, because it does not fit the requested aspect
ratio.