[XeTeX] new version of HarfBuzz

[maxwell]

I believe (correct me if I'm wrong!) that xetex uses the HarfBuzz 
engine.  There was a bug in this engine that prevented a new Nastaliq 
font from SIL from working properly.  A fix for this bug has just been 
accepted (I believe it's this one: 
https://github.com/behdad/harfbuzz/pull/475).  It's not clear to me 
whether this can get into xetex before the next TeXLive release, given 
the schedule here:
    https://www.tug.org/texlive/

    Mike Maxwell