This commit is a band aid until a more proper room setup is implemented.
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.
This patch has been spun of the rather massive amount of work for adding input
devices into Monado. The interfaces feels somewhat stable now so could do
with another pair of eyes on them, before too much work is done on st/oxr.
