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.