window: Sanitize size hint computation

window: Sanitize size hint computation

We don't want the maximum size to be smaller than the minimum size. Not
just because it's wrong but also because when this happens the rest of
GTK gets mighty confused and infloops resizing to min-size and
max-size in turns causing a flickering window. Well, at least if you
run X without a window manager. Or your window manager hasn't finished
starting up.

Private RHEL bug finding this issue:
https://bugzilla.redhat.com/show_bug.cgi?id=1035409