Correct parenting of spatial mesh to preserve local transform.

[fast-slow-still]

Overview

When the spatial mesh's transform was getting attached to the ObservedObjectParent using the parent property, the default behavior of preserving world space position was kicking in.

This sounds right, but isn't. It meant that the current ObservedObjectParent's transform (which ultimately resolves to the MRTK Playspace transform) was getting discarded (or more specifically, absorbed into the mesh's local transform).

This led to the observed behavior, that when the MRTK Playspace transform changed, existing spatial meshes remained in correct position (because they were attached to the MRTK Playspace). But new spatial meshes, because they essentially discarded the playspace transform, would appear offset from their physical counterparts.

Changes

• Fixes: XR SDK Spatial meshes partially broken with non-identity playspace #9810 .

Verification

The issue was most easily seen by clearing the hololens spatial map via WDP before running.
Only XRSDK was affected. WSA worked fine, OpenXR doesn't seem to have spatial meshes yet.

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[david-c-kline]

Nice clean fix!

[david-c-kline]

/azp run

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