Shape the glyph-string GSTRING.
Shaping means substituting glyphs and/or adjusting positions of glyphs
to get the correct visual image of character sequences set in the
header of the glyph-string.
If the shaping was successful, the value is GSTRING itself or a newly
created glyph-string. Otherwise, the value is nil.
See the documentation of `composition-get-gstring' for the format of GSTRING. (fn GSTRING)