d/steamvr_lh: simplify coordinate space conversion

I rewrote Device::update_pose() to make use of math_pose_transform() instead of doing the three operations separately, making it easier to understand what the code is actually doing. Additionally, chaperone_center and chaperone_yaw have been unified into an xrt_pose for consistency.

I have confirmed the output is identical (± floating point rounding error) to the old function.