Replace hardcoded 0 by provided constant

main.cpp

@@ -57,7 +57,7 @@ int main()
 		// [3.3] Draw phase
 		SDL_FillRect(screen, 0, SDL_MapRGB(screen->format, 0, 255, 255));
 
-		SDL_BlitSurface(grass, 0x0, screen, 0x0);
+		SDL_BlitSurface(grass, NULL, screen, NULL);
 
 		SDL_Flip(screen);