tdf#160594: Use the recommended fallback of 0.5em for ex in font-size
This fixes the error of identical treatment of em and ex in font-size,
which violated https://drafts.csswg.org/css-values-4/#font-relative-length.
The fix uses the fallback of 0.5em for ex, similar to the code used in
SvgNumber::solveNonPercentage. A follow-up should implement the correct
use of "x-height of the first available font".
Change-Id: Id9d581994e158d629d9752299ad93ac7e9fe4cad
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/166234
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>