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Stop SAD tracker when error becomes too high.
Maximal allowed error value is controlled in Tracking Settings panel.
I haven't been able to find value which will work for most of cases
so it's now quite stupid value of 0. Currently values of 2-6 gives
quite nice result, but it depends on footage.
Upcoming changes from libmv side related on returning normalized SAD
would help here. Until this, please set max_sad manually.