xrt: Add local_floor to xrt_space_overseer

Used to implement XR_EXT_local_floor.

src/xrt/include/xrt/xrt_space.h

@@ -98,6 +98,7 @@ struct xrt_space_overseer
 		struct xrt_space *root;        //!< Root space, always available
 		struct xrt_space *view;        //!< View space, may be null (in very rare cases).
 		struct xrt_space *local;       //!< Local space, may be null (in very rare cases).
+		struct xrt_space *local_floor; //!< Local floor space, may be null.
 		struct xrt_space *stage;       //!< Stage space, may be null.
 		struct xrt_space *unbounded;   //!< Unbounded space, only here for slam trackers.