Example 6 doesn't clear whole screen
On my new Windows environment, some pixels remain on the top and left side of the window when I run the example 6.

![pyxel-180825-100744](https://user-images.githubusercontent.com/678802/44613490-0d128380-a84f-11e8-8710-c1bb3c6fd90b.png)

On Mac, the example #6 works well.

