acq drift: improve comparaison of prev_drift and orig_drift

prev_drift should include the drift of the previous image to be comparable
to orig_drift.
gui overlay: speed up AR point position computation

no need to compute the minimum distance for all the points.
It's very slow and as points are uniformly distributed, looking
just at the first point is enough.

One acquisitions with a large number of AR points (eg: 30x30), it
can save several minutes.
gui widgets: use future division

as in the other files.
Fixes a problem with computing the ratio of the progress bar.
acq drift: make sure the drift is returned in SEM px

Due to the scale update, the drift unit was actually in anchor image px, which
is against the specification.
acq stream: make the estimated time for sync acquisition slightly more pessimistic

seems to actually fit better the really (for small exposure times, with the simulator)
acq stream: fix metadata of sync acquisitions

With drift correction, the position of both the CCD and SEM data was wrong.

Also use more pessimistic (aka realistic) time per pixel for the DC estimator, so that
the actual period between DC is closer to the one requested.
acq stream + gui: save anchor regions with the data

mainly for debugging purpose and for the user to be able to trust the drift correction.

also add a hack (for now) to not display the anchor region by default in Odemis.