Let the OS know the window is responsive

As seen at:
https://www.pygame.org/docs/ref/event.html#pygame.event.pump

Author: DC-fgervais

examples/mlx90640_camtest.py

@@ -143,6 +143,8 @@ def gradient(x, width, cmap, spread=1):
     img_surface = pygame.image.fromstring(img.tobytes(), img.size, img.mode)
     pygame.transform.scale(img_surface.convert(), screen.get_size(), screen)
     pygame.display.update()
+    if args.windowed:
+        pygame.event.pump()
     print(
         "Completed 2 frames in %0.2f s (%d FPS)"
         % (time.monotonic() - stamp, 1.0 / (time.monotonic() - stamp))