HFW of SEM is modified at GUI start

[pieleric]

When a SEM HFW is connected in Odemis, it might be changed at GUI start.

It seems that this is due to the SEM view (and all the other ones too) being resized for a fraction of a second to Xx1 px during startup. When this happens, the HFW is normally kept, but the mpp needed is too big, and so it's clipped. Once it's resized back to a good value, the HFW is restored based on that clipped mpp.
=> Avoid that short resize to 1px.