Note that PathGrapics uses a glyph codes as a indexes in an array
in order to provide a mapping from glyph codes back to unicode
characters. So, it assumes that glyph code is not negative.
However it is not true in case of macosx: if there is no glyph
in a font for some character, then macosx font support machinery
returns negative unicode char in order to indicate that this char
requires a substitution (CoreTextSupport.m, lines 92 - 126).
Suggested fix just ignores negative glyph codes in PathGrapics.
See also comments section.