The window is now cleared every frame

> http://sfml-dev.org/tutorials/2.1/graphics-draw.php
> Calling clear before drawing anything is mandatory, otherwise you may
> keep undefined pixels from previous frames. The only exception is when
> you cover the entire window with what you draw, so that no pixel is
> left undrawn; in this case you can avoid calling clear (although it
> won't make a big difference on performances).

> Calling display is also mandatory, it takes what was drawn since the
> last call to display and displays it on the window. Indeed, things are
> not drawn directly to the window, but to a hidden buffer. This buffer
> is then copied to the window when you call display -- this is called
> double-buffering.

> This clear/draw/display cycle is the only good way to draw things.
> Don't try other strategies, such as keeping pixels from the previous
> frame, "erasing" pixels, or drawing once and calling display multiple
> times. You'll get strange results due to double-buffering.
> Modern graphics chipsets and APIs are really made for repeated
> clear/draw/display cycles where everything is completely refreshed at
> each iteration of the main loop. Don't be scared to draw 1000 sprites
> 60 times per second, you're far below the millions of triangles that
> your computer can handle.