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.
12 jobs for master in 9 minutes and 36 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Compile
passed aarch64 #599612
docker aarch64

00:00:09

passed linux #599608
docker amd64

00:00:11

passed macos #599613
amd64 catalina

00:00:12

passed scan #599614
docker amd64

00:00:35

passed static #599609
docker amd64

00:00:09

passed win32 #599610
docker amd64

00:00:11

passed win64 #599611
docker amd64

00:00:10

 
  Test
passed gpu #599616
gpu

00:02:35

78.8285%
passed llvmpipe #599615
docker amd64

00:00:55

 
  Sanitize
passed asan #599617
gpu

00:01:55

passed msan #599618
gpu

00:02:20

passed ubsan #599619
gpu

00:01:55