Fix this bug:

Fix this bug:
1. scroll a bit so that the cursor is not visible anymore
2. hit the keyboard arrow keys to move the cursor or type something

The problem was that TextMetrics::parMetric() automatically creates a new parmetrics on const access. This was needed in the old architecture but I don't think anymore.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@23126 a592a061-630c-0410-9148-cb99ea01b6c8