FIX: RenderedStrings should get the window's Font if no explicit Font defined
This fix is related to the Fix of changeset 4f90ed9e1c70. That fix was
implemented to solve problems with Statictexts, which would use the wrong Font.
However, this caused RenderStrings to ignore the window's font. Now with this
solution this should imo solve all issues and seems the most logic resolution
of which Font to be used. Fixing issue #1097