Another soft rule: no member functions (except for the Scenes); structs are only data, all functions are free functions.

Also no operator overloading, so yes, lots of Vec3::add(&v1, &v2). I was hesitant at first but this makes for more transparent ops (* is dot or cross?) and does not hide the complexity.

The whole thing is around 6-7kloc and I think it would be possible to rewrite in C++ in a day or two.