The primary change here is that there's now an option
to skip autofocus if the background detect plugin says
the current image is background.
I have also overhauled the way it picks the next Z position
based on Joe's code; instead of just using the last point, it will
pick the closest point where we have a successful autofocus
recorded. This will usually be the last point, except for raster
scans where it neatly reproduces the behaviour of the old code
but without needing to treat it as a special case (when we jump
back to the start of a line, it will use the z position of the start
of the previous line, rather than the end of the previous line).
This does represent a minor change to previous behaviour, but
it should not break anything that isn't already broken, i.e. it might
cause slightly odd behaviour if autofocus gives random results -
but probably indistinguishable from the current behaviour.
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