a fix to the computation of the font "text height" (i.e. the
baseline-to-baseline distance). Unfortunately, we cannot rely on the OS/2 fields, like sTypoAscender or usWinAscent to compute it. Rather, we only look at the horizontal header's content. In case the "line gap" is set to 0, we compute the text height as 115% of the ascender+descender value.. (this percentage is based on the values found in many Windows fonts).