HarfBuzz is available on MS-Windows

[Eli Zaretskii]

The harfbuzz branch can now be built on MS-Windows, and will support
the new 'harfbuzz' font backend if the requisite DLLs are installed.

Work still continues on the branch in preparation for landing it on
master at a later date, but I'd like at this time to ask people to try
building the branch on Windows and report any problems they see.  Once
the branch is built, you can test that it's working by typing "C-h h"
and seeing whether HELLO displays correctly.

By default, the 'harfbuzz' backend is preferred to the other ones, so
if everything is OK, "C-u C-x =" on any character in HELLO should show
"harfbuzz" at the beginning of the font name used to display the
character.  To force Emacs to use another backend (e.g., for
comparison of the display), you can use the -xrm command line option.
For example:

   emacs -Q -xrm Emacs.fontBackend:uniscribe

will force Emacs to use Uniscribe, the previous default text shaping
engine, as the backend.

HarfBuzz DLLs are available from the MSYS2 project and from ezwinports
(the latter only for 32-bit builds).

Thanks in advance for helping to test the branch.