Hi,

please consider the attached patch for luahbtex. It adds a Lua interface
for HarfBuzz's functions around variable fonts.

It is an adoption of recent upstream luaharfbuzz changes with additional
LuaJIT support. While corresponding busted tests are included too, the
font needed to actually run these tests has not been added to the fonts
directory since sending binary files in diffs via email is problematic
and I don't think that anyone is running these tests from the LuaTeX
repo anyway.