[MRTK3] Adding support for unbounded tracking mode

[AMollis]

Overview

The Microsoft.MixedReality.OpenXR.EyeLevelSceneOrigin component in Microsoft's MR OpenXR plugin will soon be deprecated (so to avoid having mono behaviours within this plugin). Also, Unity doesn't have UI to set the tracking mode to TrackingOriginModeFlags.Unbounded on the XRInputSubsystem.

So, to replace Microsoft.MixedReality.OpenXR.EyeLevelSceneOrigin, an UnboundedTrackingMode MonoBehaviour has been added to MRTK3. This behavior has been applied to MRTK's XR Rig and will force HL2 into unbounded mode if Unity's XROrigin.RequestedTrackingOriginMode is set to "Not Specified". If the platform does not support unbounded mode, the new behavior does nothing.

Unbounded mode is preferred on HL2 (the only device currently supporting this mode).

For more information on unbounded mode, see the Microsoft OpenXR extension XR_MSFT_unbounded_reference_space @ https://registry.khronos.org/OpenXR/specs/1.0/html/xrspec.html#XR_MSFT_unbounded_reference_space.

Additional Note

You may ask "why is this using XRLoader.GetLoadedSubsystem and not SubsystemManager.GetInstance to find the input subsystem?" Well... Holographic App Remoting (HAR) apparently connects later than the physical device. So, on HAR the subsystem manager doesn't have input system handy by the time UnboundedTrackingMode is loaded and when it needs to listen for input events.

While all on the other hand, XRLoader has the input system handy at start.

It looks like XROrigin worked around this by re-querying for the input system every frame, until it finds one. However, since MR Plugin is already requiring UnityEngine.XR.Management and this method has been tested and used by the MR Plugin, I'm going to stick with XRLoader for now. The dependency on UnityEngine.XR.Management is not mandatory for MRTK Input, so this usage shouldn't break other platforms.

Changes

• Fixes: Add version of Microsoft.MixedReality.OpenXR.EyeLevelSceneOrigin to MRTK3, as the MR Plugin version will soon be deprecated and Unity doesn't have UI for setting Unbounded Space #11749

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