rawspeed: proper Samsung NX300 (and family) cropbox

Samsung has a weird thing going on where the center of the
sensor isn't the center of the image when processing a RAW.

So let's take the largest possible 3:2 cropbox.

This has been tested with the NX300, and also applied to all
Samsung NX camera's with a 2.5 gen sensor.
wb_presets: Samsung NX200 family copy