Calculate the font size correctly even when the size is specified
in pixels and update this size when the font changes.
Also make sure that the text layout's font is set before parsing.

Task-number: QTBUG-24458

Change-Id: Ida7723f6e4f4b9fd3a6878076f4beaf5bda8f7f7
Reviewed-by: Andrew den Exter <andrew.den-exter@nokia.com>