driver semcomedi: fix placement of pixel centers

The e-beam position of each pixel must be at the center.
The computation of the coordinate was wrong, leading to the ebeam being placed almost
at the border of the pixel. This was affecting very large scales, such as when doing fine alignment.

Note that spot mode was not affected.
acq fine overlay: don't compensate (twice) for binning when computing the scale correction

the binning is already taken into account for the pixel size computation, so no need to divide by it again.