Segmentation.hpp: adapt the code to OpenCV 3

In OpenCV 3 the public cv::BackgroundSubtractorMOG2() is now abstract so
we cannot inherit directly from it.

The concrete class would be BackgroundSubtractorMOG2Imp from
modules/video/src/bgfg_gaussmix2.cpp but it is private so we cannot
inherit from it either.

Use the factory method as documented to get a cv::BackgroundSubtractor
instance of the desired type and use is as a property in the
MOG2Segmentation class.