I come bearing gifts.
I have managed to get mupen64plus_next to compile - and I got it to work as well. So now retroarch has N64 support.
Now, I will not be making a PR for this branch. It is built on an older version of mupen64plus_next that does not use threaded gfx plugins. Currently, the threaded version compiles, but fails to work. I will have to figure out something.
However, in the meantime, this plays N64 games just fine!
(As for flycast textures - to be brutally honest, I’m not sure what’s going on there. I may have to take a closer look later)