Cropping of correlation map

Correlation map of F1 and F2 needs to be cropped to get rid of the
correlation coefficients resulting from the padding. Like this, only the
meaningful correlation coefficients remain in matrix. Cropping value =
twice the padding applied before as this should roughly be the maximum
amount of "non-meaningful" pixels attached. And the meaningful value,
the max corr-coefficient lies somewhere in the middle of the picture. So
it's safe to crop twice the padding.