[XRI3] [BUG] Spatial mouse manipulation are relative to scene origin instead of head pose

[AMollis]

To reproduce

Steps to reproduce the behavior:

1. Build sample from Features/XRI3 branch for HL2
2. On HL2 go to SpatialMouseSample.unity scene
3. Connect blue tooth mouse to scene.
4. Move away from origin (move to the left or right of scene content)
5. Select an object in scene, use mouse scroll wheel
6. Result 1: Content scrolls perpendicular to head gaze (left-right), instead of parallel to gaze (backward-forward)
7. Rotate scene content 90 degrees around y axis by grabbing description panel
8. Select an object, and move mouse up and down
9. Result 2: Content does seem to move correctly (also the mouse pointer has a hard time intersecting with scene content)

Expected behavior

1. Result 1: Content to scroll parallel to head gaze (backward-forward)
2. Result 2: Content to move up and down and can be targeted easily with mouse pointer.

[MaxPalmer-UH]

Not reproducible in editor on Windows using this branch/scene. Will check when using a bluetooth mouse with a Quest headset. NB HoloLen2 is no longer a test target for MRTK.

[MaxPalmer-UH]

[MaxPalmer-UH]

@AMollis On point 7. is the rotation of 90 degrees achieved with a movement on the mouse with the description panel selected? I can't see how to do this using the controls.

[MaxPalmer-UH]

So, paring a bluetooth mouse on the Quest 3 works and mouse events for position moved (mouse delta) and scrollwheel changes are received in SpatialMouseInteractor.cs. However, the default viewport that the mouse movement is mapped to appears to be a sort of horizontal plane / volume that sits below the content in the sample scene so no interactions with scene content are possible. I'll look at fixing that bug as part of this work instead.

[MaxPalmer-UH]

Fixed in the above PR. The parent has been changed to be the main camera, which fixes the mouse origin issue and now movement and interaction is with respect to the head origin and looks/works correctly.

[keveleigh]

Fixed in #980