VR: SteamVR Seated Mode support

Currently the VR Scene Inspection runs in standing mode in SteamVR, which makes the chaperone grid render when outside of the designated playspace. This is useful as a safety feature when walking around the room, but since the computer can't be inside this area, the chaperone will always be visible when using the VR inspection while sitting at the computer. When on, the chaperone renders in front of the scene, so it actively interferes with viewing the model.

Using the inspection while seated is probably going to be the most common use case when working with Blender, so it would be a good idea to have a way of disabling the chaperone only when seated.

SteamVR supports a seated mode, where the chaperone is disabled while the HMD is withing 1m of a user specified tracking origin. I do not know if this feature is available from the OpenXR API though.

I think the best way to implement this would be a toggle between the seated and standing modes in the VR Session panel, along with a button to reset the seated tracking origin.