It would be a pretty fun project to write a CPU software rasterizer that works in UEFI… (Obviously GPUs are out of the question, but it might be possible to use multiple CPU cores)
Just scrolling a panorama along would probably look good enough
I think the real game uses a cubemap so the camera is inside a cube with six textured faces which I guess should be easy enough to render if you wanted to do it properly
Is that even possible in Grub? Last time I checked there was a tool called "Burg" that would animate backgrounds like a gif, but otherwise it was very clunky and not a great experience.