demos: add some fflushes for windows

progs/demos/streaming_rect.c

@@ -153,6 +153,7 @@ static void Display( void )
 
       GLfloat fps = Frames / seconds;
       printf("%d frames in %6.3f seconds = %6.3f FPS\n", Frames, seconds, fps);
+      fflush(stdout);
 
       drift_increment = 2.2 * seconds / Frames;
       T0 = t;