Function histogram_add

deps/opencv/modules/imgproc/src/smooth.cpp:933–938

static inline void histogram_add( const HT x[16], HT y[16] )
{
    int i;
    for( i = 0; i < 16; ++i )
        y[i] = (HT)(y[i] + x[i]);
}

static inline void histogram_sub( const HT x[16], HT y[16] )
{