initial. copy from XrHandsFB.
OPENXR_SDK_DIR
=> OpenXR.Loader.1.0.25.nupkg or https://github.com/KhronosGroup/OpenXR-SDK (use openxr.h)OVR_MOBILE_SDK_DIR
=> ovr_openxr_mobile_sdk_46.0.zip from https://developer.oculus.com/downloads/package/oculus-openxr-mobile-sdk/ (use libopenxr_loader.so)