st/oxr: Add env vars for global tracking origin offset
This commit is a band aid until a more proper room setup is implemented. It is not how we really want to do this, but it is quite useful in allowing basic user side 6dof setup that is important for certain applications. OpenMW for example is not really playable when the user is not at the right height.
It allows moving the tracking offset for the device roles head, left and right by a fixed value.
A y tracking offset OXR_TRACKING_ORIGIN_OFFSET_Y=1.0 would tell monado that the current tracking origin is 1 meter above the desired tracking origin, e.g. when the headset was calibrated to a (0,0,0) position while sitting on table 1 meter above the floor.
This environment variable affects STAGE space, but not LOCAL space.