testcard gray pattern: decrease default step to 1

The previous value 16 was too much fast (one iteration lasted only 16
frames) which was even unsuitable for latency evaluation for higher
latency compressions.

With the value 1, the latency can be directy read from difference of
luma on subsequent lines.

src/utils/video_pattern_generator.cpp

@@ -563,7 +563,7 @@ struct gray_video_pattern_generator : public video_pattern_generator {
                 return out;
         }
 private:
-        constexpr static int DEFAULT_STEP = 16;
+        constexpr static int DEFAULT_STEP = 1;
         int step = DEFAULT_STEP;
         int width;
         int height;