Clearing out old branches from when I was working on getting DOOM 2016 up and running. It requires this extension, and it's pretty trivial to implement. Since the software present path doesn't have any concept of actual VBlank synchronization, it always does the tearing behavior, so the flag is essentially ignored. The d3d12 DXGI winsys also ignores it because there's no such thing as -0, which would be the only way that DXGI can make an interesting decision based on this extension. There's no way to express "wait for at least 1 VSync but then don't wait for any more and tear."
I also added some debug printfs that were useful when debugging.