REALLY update screen only every 10th line

mandelbrot.c

@@ -52,7 +52,7 @@ void drawmandelbrot(SDL_Surface *surface) {
     pixels[i] = color;
 
     /* Update the screen every 10 lines. */
-    if (y % 10 == 0) {
+    if (y % 10 == 0 && x == 0) {
       SDL_Flip(surface);
     }
   }