Autofocus timing is pretty critical. I've introduced the ability
to abort moves, which introduces ~50ms timing jitter. This
commit reverts the tming-critical move in autofocus to the old behaviour.
I've only reverted the move that really matters - this means
that if you start a massive sweep by accident, you can still
abort it when it's moving down to the start of the sweep.
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.
