fdo#74555: Avoid excessive calls to GetPreviewCellStyle().

GetOptimalHeight() gets called *all the time* and is very performance
sensitive.  Let's not do an expensive operation such as GetPreviewCellStyle()
which slows down pretty much everything else after the non-default font is
applied.

Change-Id: I5d5b2e706dd5dd98173044642b98ccaedd9536f0
(cherry picked from commit 947177a5cb79e03a415d9c3d1203420c874d2b35)
Reviewed-on: https://gerrit.libreoffice.org/9074
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>