Actually, I completely misunderstood the original post - that was the wrong link. The repo is at https://github.com/libretro/Genesis-Plus-GX-Wide .
Here are the instructions to install this:git clone --recurse-submodules https://github.com/libretro/Genesis-Plus-GX-Wide cd Genesis-Plus-GX-Wide make -f Makefile.libretro
Then you may copy the generated genesis_plus_gx_wide_libretro.so file to a folder to use it. What I did in my case was:sudo mkdir /opt/retropie/libretrocores/lr-genesis-plus-gx-wide sudo cp genesis_plus_gx_wide_libretro.so /opt/retropie/libretrocores/lr-genesis-plus-gx-wide/
and then added it to /opt/retropie/configs/megadrive/emulators.cfg , in this case aslr-genesis-plus-gx-wide = "/opt/retropie/emulators/retroarch/bin/retroarch -L /opt/retropie/libretrocores/lr-genesis-plus-gx-wide/genesis_plus_gx_wide_libretro.so --config /opt/retropie/configs/megadrive/retroarch.cfg %ROM%"
If it doesn't compile at first, you may need the following dependencies:sudo apt install libopusfile-dev sudo apt install libjansson-dev
Then, in-game, adjust the "columns to add for widescreen" option. See recommendations in the original link.
To be honest, it is very experimental and, to be honest, a bit more "this sounds awesome in concept, but in hindsight it really becomes clear why this wouldn't really work". Sure, we get a wider viewport but it is often rendering things that are meant to be hidden from the user (so, when not in the game, it's placeholder text or gibberish). Also, even though in-game the viewport is bigger, most games don't render sprites there, meaning that animated enemies will not show in the "overflow"/"wider" area, only the background layer, or there will be massive pop-in. And, of course, you cannot move there.
But as an interesting experiment, it's there - by all means try it out.