SLAM: default timing is not written when there's no timing extension
c2009b6a gated the TimingWriter::push
behind t.timing.ext_enabled
(https://gitlab.freedesktop.org/monado/monado/-/blob/main/src/xrt/auxiliary/tracking/t_tracker_slam.cpp#L824) so we no longer get the default timing (https://gitlab.freedesktop.org/monado/monado/-/blob/main/src/xrt/auxiliary/tracking/t_tracker_slam.cpp#L1404) written to disk.