Clamping the anchor values as introduced in commit 9a5ffcd1 to fix bug
777176 breaks menu positioning.

By keeping the anchors rectangle size greater than zero, we end up
deducting some positive value from the original position, so there is no
need to clamp() actually, keeping the values positive is enough and
avoids the issue with menu positioning on the menubar.

An additional benefit is to make the code a lot simpler.

https://bugzilla.gnome.org/show_bug.cgi?id=778009