Correct generation of ghost hints for references

When FontForge autohints a glyph that is a reference to some other glyph, ghost hints in the original glyph become ordinary hints in the reference. As a result, during OpenType/CFF font export these hints are written with positive stem widths. The problem is that in OpenType/CFF ghost hints should have negative stem widths ("The Type 2 Charstring Format", page 6).

This patch fixes this behavior.