Globally enables or disables VR for the application.
When enabled, we are accepting input from the device and rendering to the device. This can become false if a device is disconnected or a device could not be intialized (see VRSettings.LoadDeviceByName).
VR output is automatically mirrored to the main display (if applicable). This can be controlled with VRSettings.showDeviceView.
The main window is still controlled by Screen and related APIs.