window: Add a new fancy way to compute min size

window: Add a new fancy way to compute min size

Try to compute a min size that matches the current aspect ratio.

This means that when interactively resizing, we adapt the min size to
the current window area dynamically.

And that means that we always have a min size that is large enough, but
users can interactively cause it to be small-width x large-height,
large-width x small-width or anything inbetween.