As someone who's unfamiliar with how libretro works (and with only passing knowledge of RetroArch), what's the difference between this PR and your work over at libretro/scummvm? Does that repository not already provide what's needed to make a ScummVM core for RetroArch?

I'm not a member of the ScummVM team, but a PR description may help in case team members have that question as well.

There is no difference with current libretro core. It's same sources rearranged to fit ScummVM tree (while current core sources are structured as a wrapper with a scummvm git submodule). Purpose is to eventually drop current core repo and directly use ScummVM official repo instead.

This has been discussed with ScummVM Team in advance.

Hopefully the code here keeps being worked on until at least the opengl renderer works, not just keep building. That was always the worst loss on libretro-scummvm, especially for 3d games. Although i guess that's 'difficult' if you want it to happen on the console implementations of opengl.