Support Harfbuzz

[deepakjois]

Would you consider supporting Harfbuzz in mmtex. LuaTeX already supports it (when compiled with an optional flag), and so does luaotfload. I imagine the changes in this repo would be to download some additional source files and then build them.

[vlasakm]

Hello, I already tried it with this commit here: 9c7e2ea (currently in not updated branch luahbtex). I dismissed this initially for a few reasons:

1. OpTeX format (currently the only format in MMTeX) does not have "extra" support for harf mode, this is noted by the documentation http://petr.olsak.net/ftp/olsak/optex/optex-doc.pdf (section 2.13.14 Notices about extension of \font primitive). But it is possible for the user to add font features (e.g. section 2.13.9 Modifying font features), so it is possible to leverage harf mode.
2. I was compiling Debian binaries on Debian stable (to support it and also newer/derived systems). However the harfbuzz version included there (2.3.1) is too old for LuaHBTeX. But new Debian stable (Bullseye) should be near and has a relatively new version.

However because luahbtex can be leveraged in OpTeX, and because old Debian is probably not going to matter soon I added the support now (see latest commits). It is compile time optional, but default. The system harfbuzz library is used. I am not going to do a release yet (waiting for offical LuaTeX 1.13, fix of probles with hyphenation patterns, both will happen probably only after TeX Live 2021 release).

Still, this doesn't change that OpTeX in TeX Live and MikTeX is based on luatex. The acknowledgment of luahbtex in OpTeX documentation comes from my brief discussion with Mr. Olšák about it. Considering that building the format on top of luahbtex shouldn't have any side effects unless harf mode is set, maybe he can be persuaded to allow change of underlying engine. And these distrbutions already have to solve harfbuzz for lualatex anyway.

[deepakjois]

Thanks for additional context, and for making the change.

And yes, I do agree that changing the underlying engine from luatex to luahbtex should have no regressions. Everything that is currently working with OpTeX will continue to work. So there isn't a strong reason to not make that change, given that it provides the additional benefit of being able to use Harfbuzz.