Game of Life was going to be my next idea for a (nightly) Fortran SDL project. I’m glad you’ve done it already. 
I also created a custom SDL2 backend in C with Fortran bindings for the Fortran CHIP8 virtual machine. SDL2 was easier to use than Xlib. Initially I tried using EGGX/ProCall but it lacked the functionality to distinguish key releases from key presses.