pyxel.cls(0) is not clearing the screen properly for some resolutions
Some pixels do not get overwriten(/overdrawn) on some resolutions.
```
import pyxel

class MyGame:
    def __init__(self):
        pyxel.init(100,100)
        self.x = 0
        pyxel.run(self.update, self.draw)

    def update(self):
        self.x = (self.x + 1) % pyxel.width

    def draw(self):
        pyxel.cls(0)
        pyxel.rect(self.x,0,self.x+7,7,9)

MyGame()
```
this code leads to this result:
![100x100](https://user-images.githubusercontent.com/41760497/43658073-0e5e68a6-9758-11e8-90cc-0d61b3aca0e2.gif)

but if I change the pyxel.init() to
```
pyxel.init(100,101)
```
the result is fine:
![100x101](https://user-images.githubusercontent.com/41760497/43658121-37cfa880-9758-11e8-81ad-46b1b7288007.gif)

This code shows how the bottom and right border aren't drawn properly
```
import pyxel

class MyGame:
    def __init__(self):
        pyxel.init(100,100)
        self.x = 0
        pyxel.run(self.update, self.draw)

    def update(self):
        self.x = (self.x + 1) % pyxel.width

    def draw(self):
        pyxel.cls(13)
        pyxel.rect(self.x,0,self.x+7,7,9)

MyGame()
```
![pyxel-180803-203905](https://user-images.githubusercontent.com/41760497/43659780-4bf34312-975d-11e8-89f3-95c1345b749f.png)

- Win10,
- Python 3.6.2
- pyxel 0.7.4