Fix for DirectX 12 with SteamVR

I’ve got a pull request fixing dx12 use with Steam VR:

It should fix the crashes and hangs that could happen before (Unreal Engine Issues and Bug Tracker (UE-78362)).

I saw a little bit better performance with it vs dx11 (980ti in 4.23), and one other person saw a little bit worse performance vs dx11 (gtx 1000 series using 4.22).

If the fix works for you I’d interested to hear how it affects your performance compared to dx11 as well (and what what kind of card you are using).

What would be the best way for me to try to incorporate this fix into my build? (I am a bit new to UE4, etc).