Update Viewports tutorial to use "frame_post_draw"

viewport/screen_capture/screen_capture.gd

@@ -4,9 +4,8 @@ onready var captured_image = $CapturedImage
 
 func _on_CaptureButton_pressed():
 	get_viewport().set_clear_mode(Viewport.CLEAR_MODE_ONLY_NEXT_FRAME)
-	# Let two frames pass to make sure the screen was captured.
-	yield(get_tree(), "idle_frame")
-	yield(get_tree(), "idle_frame")
+	# Wait until the frame has finished before getting the texture.
+	yield(VisualServer, "frame_post_draw")
 
 	# Retrieve the captured image.
 	var img = get_viewport().get_texture().get_data()