bnc#882631: Do not hide top of the first row when editing text in table cells.

This can happen when proportional line spacing is less than 100%
and "FontIndependentLineSpacing" is set, so just keep the height
of the very first line in case of IsFixedCellHeight() too.

Change-Id: Id231fc1f2373645e5420f877e8c2568ce8a1f099