It's helpful to raise an exception if we access the calibration
before it's been set - but thing state should not raise exceptions,
just return an empty dict if the data is not there yet.
Currently only has fast_autofocus, and there
may be some work to do on timing, but it works :)
Getting timestamps from the camera rather than
time.time() would be ideal, but it would take some thought to tie that up with
the encoder.
I'm using a low-res stream, as it seems impossible to
turn the bitrate control of the MJPEG stream off,
at least within the picamera2 API.
