Experimental Firestorm Viewer + P373R VR Mod source build

This build includes P373R's VR Mod 6.3.3 changes merged into Firestorm 6.4.12 release branch.

Change Set: Firestorm_6.4.12...humbletim:gha/Firestorm_6.4.12_VR

See http://blog.inf.ed.ac.uk/atate/2020/12/11/firestorm-vr-mod-6-4-12/ for Austin's overview on using VR mode.
See https://gsgrid.de/firestorm-vr-mod/ for more information on Peter's VR Mod.

Notes:

• App name has been changed to "FirestormVR" so installation can exist side-by-side with stock Firestorm
  • settings stored in user folder AppData/Roaming/FirestormVR_x64