Bugfix :
getFaceList works on the main image, so we use a ratio between main/current
image to set "safe" zone list
Change the "getLimit" method to "getPotential" method
This method now return the "potential" of a slice.
The potential is the area of all safe zones in a slice.

We can now compare two "safe slides" to avoid blocking on very small heads instead of large head

The return value of getPotential depends on the image size, we can compare two slices, but we can not extrapolate some constant value.