[Dev-luatex] [PATCH] luaharfbuzz: Provide interface for variable fonts

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. Best regards, Marcel