Scene Understanding improvements

[shukenmg]

This PR adds these features:

1. Spatial Mapping can now use XR_SCENE_COMPUTE_CONSISTENCY_SNAPSHOT_INCOMPLETE_FAST_MSFT for mesh data.
   This results in a smoother and more complete mesh, trading off speed for quality.

2. Fixes Unnecessary Plane Updates when using spatial mapping. #102

3. Fixes Support LOD and better spatial mesh quality in Scene Understanding #101
   Support for LOD, defined in https://registry.khronos.org/OpenXR/specs/1.0/man/html/XrMeshComputeLodMSFT.html

4. Changed SpatialMeshingVolumeSize from being a platform limited variable to being available anywhere, this is to support Holographic Remoting. Also, renamed it to SceneUnderstandingVolumeRadius

5. New config variables to control the features above
   ShouldDoSceneUnderstandingHighQualityMeshing controls the spatial mapping consistency
   SceneUnderstandingVolumeRadius renamed and relocated from SpatialMeshingVolumeSize
SceneUnderstandingMeshingLOD this is a interger based value corresponding to the XrMeshComputeLodMSFT definition.

6. Also, there is this issue where the meshes/planes won't be updated the first time because its UnderlyingMesh wasn't ready, this PR fixes this by allocating new meshes/planes during ProcessSceneUpdate so that its UnderlyingMesh will be ready for the mesh/extents data update.

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[shukenmg]

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