[NTG-context] Rendering of ZWNJ and ZWJ

Using Luatex engine, ZWNJ and ZWJ chars are rendered incorrectly. If the font has a glyph for it, it does the expected behaviour (force non-joining or joining respectively) but the glyph get rendered while it shouldn't. But if the font has no glyphs for it, it has no effect at all. See the attached example. -- Khaled Hosny Arabic localizer and member of Arabeyes.org team