Add Spatial Window mode

[AJBats]

This PR adds a spatial window mode to UEVR that gives users a "fishtank" style view.

Motivation:
I kept getting motion sick in Stray, and wanted to keep playing in a more comfortable way while keeping the "real VR 3D" view. After I built a small prototype I realized this is a great way to slowly acclimate players to full immersive VR. It also offers a way to reduce raster cost, will be handy to midrange PCs.

Features

1. The spatial window is uses the same VR 3D projection as full immersive 3D but confined to the UI plane. Head movement parallax will be observed in the scene.
2. Main control slider is "Spatial Size" which defaults to 1.00. Players can reduce the spatial size to increase comfort. This will shrink the world to maintain FOV.
3. For "decoupled pitch" added "Enabled Spatial" separate from "Enabled VR". I noticed while playing stray is was fun to pop between spatial and VR which each benefited from different pitch settings.
4. Compatible with all rendering modes, native / sequential / AFR.

Question to maintainers
Is this something interesting maintainers would be interested in for UEVR? It works well enough for my own purposes, but it needs a lot more testing. I dont have any dx12 games atm. I'll happily by one and increase the scope of my testing if folks are into this feature.

Tested working so far:

1. PSVR2 OpenVR and OpenXR
2. Oculus OpenXR
3. Virtual Desktop SteamVR + OpenXR UEVR.

Known Issue

1. Virtual Desktop VDXR reslts in double image the same as 2D view
2. I don't have any dx12 games so dx12 is untested.