Retrieving metadata pre-action risks stale metadata being
returned if DirectThingClients are used. I've switched to
the safer callable dependency, which is guaranteed to be fresh.
Previously, we raised an exception (and thus HTTP error
code) if the CSM matrix was requested and not yet set.
Now, we just return None, and I've added a (not-network-exposed)
function that will raise an HTTPException if it's missing.
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.
