demos: build both glfw and sdl
Apparently GLFW has a habit of not working on Wayland, because they hard-code the platform at compile time. Might as well just provide all four variants, who really cares about a few extra binaries? The proper solution would be to make loading the windowing system dynamic at runtime, but that probably requires handling command line arguments and I can't really be bothered. Also suppress a warning in the sdl code by rewriting it slightly.