retinex: Change retinex to C++

retinex: Change retinex to C++

Change the gstretinex.c file to cpp and add it into Makefile.
It is necessary to migrate the retinex element to C++,
because new Opencv API leaves obsolete functions like cvSmooth.
This element uses this function.

You can see in this link:
http://docs.opencv.org/modules/imgproc/doc/filtering.html?
highlight=cvsmooth#void cvSmooth(const CvArr* src, CvArr* dst,
int smoothtype, int size1, int size2, double sigma1, double sigma2)

https://bugzilla.gnome.org/show_bug.cgi?id=754148