Fix bug #7662: Incorrect scrolling for rows with larger ascent.

Fix bug #7662: Incorrect scrolling for rows with larger ascent.

- After introducing the displayed vertical alignment for tables, we met for the first time situations in which row.ascent is very large. To make sure the scrolling still works, we have to do the same as we did for rows with a large descent.
- change row.ascent into defaultRowHeight. Before the above mentioned change, row.ascent usually was approximately the same as defaultRowHeight (by chance). However, it is now clear we don't really want row.ascent here.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@39486 a592a061-630c-0410-9148-cb99ea01b6c8