HarfBuzz is now available on master

[Eli Zaretskii]

I've landed the harfbuzz branch.  If you have the HarfBuzz library
installed, your next build of the master branch should use HarfBuzz as
the text shaping engine in preference to all the others.

There's one issue that needs to be discussed in this respect, before
we finalize the HarfBuzz support; I will start a thread here in a
moment.

Let me take this opportunity to thank all those who participated in
the effort: Khaled Hosny who wrote the initial code for Posix
platforms, Mitsuharu Yamamoto who refactored that code into a separate
font back-end and made other important improvements, and Behdad
Esfahbod and all the rest of HarfBuzz folks who helped by answering
many of my questions, in particular about making the new backend work
on MS-Windows.

We can now consider our text shaping future to be in good hands.