util img: special trick in findOptimalRange to skip full blacks

Sometimes (non-rectangular) images get unknown values replaced by 0s.
That can be a lot of 0s, which completely mess up with the outliers computation.

Detect such a case, and then skip the 0s. This improves a lot the contrast for
instance on tiled images.
dataio: fix test case for new lossy format
Overview thumbnail bug fix
Added info bar to Overview Viewport
History clear when venting chamber
Added dynamic view size indication in the Overview canvas (when the SEM stream is active)
Merge branch 'master' into kimon-MiniCalibration
Parent connection in beam_blank
Merge branch 'kimon-MiniCalibration' of vpn.delmic.com:odemis into kimon-MiniCalibration