[Groff] bug in hpftodit

[Werner LEMBERG]

I've just fixed a bug in hfptodit which seems to be there since ever.

Contrary to the PCL 5 developer's guide, ascent values can be
negative.  On Colin's image 3.jpg you can see a large amount of
whitespace above the `_', because the height of the underscore is
ridiculously high: A signed 16bit integer has been read as unsigned.

This is fixed now; consequently, I've regenerated all LJ4 font
description files.


    Werner