Multipoint 6DoF behavior doesn't work correctly in XR

[RaananW]

When in XR multipoint scaling of the 6DoF doesn't work correctly. The mesh either scales incorrectly or moves to a different position when one of the hands is triggering a pointer up.

An initial solution can be found on my local branch - https://github.com/RaananW/Babylon.js/tree/6DoFMultipointFix-Start

The main issue is that in XR the transform nodes used during drag/scale/move are parented to the origin mesh (usually the controllers). This work correctly, but the reset function doesn't work correctly because it doesn't take this parenting into account. Another issue is that during PointerDown there are different state changes that do not happen in the reset function.

To avoid making changes that might break other scenarios I am not about to commit these changes or continue developing the fix for now, and will wait for the 7.0 release to fix it.

Note that this PR is merged for 7.0 - #14911 , which disabled multipoint behavior in XR scenarios.

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