u/space_overseer: skip actually locating spaces in themselves

It is always going to be identity. Keep calculating the offsets though.

Part-of: <https://gitlab.freedesktop.org/monado/monado/-/merge_requests/2194>

src/xrt/auxiliary/util/u_space_overseer.c

@@ -593,7 +593,13 @@ locate_spaces(struct xrt_space_overseer *xso,
 		struct xrt_relation_chain xrc = {0};
 
 		m_relation_chain_push_pose_if_not_identity(&xrc, &offsets[i]);
-		build_relation_chain(uso, &xrc, ubase_space, uspace, at_timestamp_ns);
+
+		// crude optimization: If locating a space in itself, we don't actually need to locate the space itself.
+		// only the offsets need to be applied.
+		if (spaces[i] != base_space) {
+			build_relation_chain(uso, &xrc, ubase_space, uspace, at_timestamp_ns);
+		}
+
 		m_relation_chain_push_inverted_pose_if_not_identity(&xrc, base_offset);
 
 		// For base_space =~= space (approx equals).