Making differences between floor-offset-relative space and rig-relative space explicit

[Zee2]

Overview

So far in MRTK3, we've used device-relative coordinates for everything, with the head always starting at 0,0,0. However, XROrigin and Unity XR in general have two different "origin tracking modes": Floor and Device. In Floor mode, trackables are reported relative to the precalibrated floor elevation; i.e., the coordinate space is absolute. In Device mode, trackables are reported relative to the point in space at which the HMD was first detected. Some devices support both (Quest, VR in general), and some devices only support Device mode (HoloLens, etc).

This PR makes our rig respect both modes. By default, the XROrigin is now set to Unspecified mode, with a default backup floor height of 1.6m. On VR devices with precalibrated floor heights, the device pose(s) will be reported with absolute coordinates, and the Camera Offset object will have an offset of (0,0,0). On devices that only support Device mode, the 1.6m backup offset will be applied to the CameraOffset object.

The sample scenes have been re-authored accordingly; all content is now placed at "head height" (1.6m). This is a breaking change, as downstream users will have to re-center their app's content to use appropriate heights/positions. (The alternative would be to set their XROrigins to Device mode only, with the awareness that this may break locomotion or avatars for VR content).

Fixes #11017.

Changes

• Deprecates PlayspaceUtilities.ReferenceTransform.
  • Users should now use either PlayspaceUtilities.XROrigin.CameraFloorOffsetObject (for AR trackables like device poses, anchors, and meshes) or PlayspaceUtilities.XROrigin.Origin for checking where the user's rig (0,0,0) is.
• Moves XROrigin and ARSessionOrigin onto the CameraOffset object itself
  • This is so that AR trackables like anchors, meshes, planes, and trackable devices are placed relative to the OpenXR scene-origin-space.
• Adjusts all sample scene content to be absolutely positioned for head-height.
• Removed extraneous ProxyInteractor from InputFieldExamples
• Removed extraneous ProxyInteractor from VirtualizedScrollRectList example scene
• Fixed LoFiAudio example scene (UI was broken and unresponsive)
• Fixed the SpatialMapping example scene (was previously broken due to missing mesh manager), and ensured it works with the new rig hierarchy
• Added more comprehensive description to SpatialMapping example scene
• Fixed wonky Gaze example scene materials
• Added TransformPose, InverseTransformPose helpers to PlayspaceUtilities
• Fused most uses of position + rotation in InputSimulator and simulated devices into using Pose structs
• Fixed misleading naming in InputSimulator (SimulatedInputPosition/Rotation -> CameraRelativePose)
• Fixed incorrect coordinate space transformation in PolyfillHandRayPoseSource
• Fixed incorrect coordinate space transformation in ArticulatedHandController (devicePose was assumed to be global)

Verification

All existing unit tests pass. Would appreciate verification from the rest of the team testing other VR devices.

[RogPodge]

Did a review of the code files and those look good to me.

[david-c-kline]

Looks reasonable to me

[keveleigh]

Didn't really look at all the scenes this time around, but the rest seems reasonable!