Fix bug 3427:

Fix bug 3427:
http://bugzilla.lyx.org/show_bug.cgi?id=3427

The problem was that offset_ref_ was calculated based on an empty metrics. The solution is delay the calculation up until the next metrics update.

* BufferView:
- center(): now just set the anchor_ref and program a new screen recentering.
- updateOffsetRef(): update the offset_ref_

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@19721 a592a061-630c-0410-9148-cb99ea01b6c8