Exit condition on 1st iteration

- If the displacement is smaller than the minimum displacement on the first iteration, we do not exit the loop, since we consider it takes more than one iteration with more than the minimum displacement allowed to register two images.
- We choose to do this in precaution of an initial step computed (alpha0) too small, that would therefore need a small displacement that would respect the exiting condition.
- If we keep this, we could even choose not to compute the displacement value for the first iteration. We still do it here to be able to see the value it has been moved and continue the debugging.