Setting a fixed default font height parameter doesn't make much sense when
neither the target device nor its mapping mode nor its target resolution are
known. This change reverts revision 1589449 of bug 124686 that has already been
solved differently and restores the previous behaviour so that the default height
of default constructed fonts is set to 12pt when the target device becomes known.
Reviewed-by: Armin Le Grand <alg@apache.org>