sanitize user/fullscreenable condition

whether or not a window is maximizable does really not
matter (it's not if the size is locked, so the
maximized size is the only size) but only if the
window can take the fullscreen geometry.

otoh, the size restrictions (maximum size, fixed aspect)
should be honored by default but still ignorable by rule


FIXED-IN: 4.11.2
REVIEW: 112654