struct face question

[Po Lu]

Is it ever valid for face->fontset to be -1?  The X font backend doesn't
support scaling fonts, so displaying the separator-line face fails to
find a font with height 0.1, causing the following part of
realize_gui_face to be called:

      else
        {
          face->font = NULL;
          face->fontset = -1;
        }

leading to face_for_char aborting from face->fontset being -1 later on.