[RFC] Cut fold and sign columns at end of buffer

```
win_draw_end() is used to clear the end of the screen in certain cases,
e.g. very long, wrapped lines or the actual end of the buffer.

Fold and sign columns get continued although that's not useful at all
for the case of the end of buffer.

Therefore win_draw_end() gets split into:

    draw_end_of_screen()
    draw_end_of_buffer()
```

Before: ![before](http://i.imgur.com/WbdtvMi.png)

After: ![after](http://i.imgur.com/Pp6FtqR.png)