minor SURF_GPU optimization (descriptor calculation, change block size to 6x6)
minor SURF_GPU optimization (descriptor calculation, change block size to 6x6)

opencv/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,apavlenko/opencv,apavlenko/opencv,opencv/opencv,apavlenko/opencv,apavlenko/opencv,opencv/opencv