Games do not render correctly beyond 256x256 resolution.
If you pass a width or height to `pyxel.init` that is above 256, the rendering is broken. The background color does not fill the screen properly (there is a black margin around the top and sides).

Above 256x256, the `pyxel.cls` command does not work correctly either. Things just blit over each other, so you can still see artifacts from previous frames.

At Line 40 in `constants.py`, two constants are defined:

``` python
RENDERER_IMAGE_WIDTH = 256
RENDERER_IMAGE_HEIGHT = 256
```

That code is presumably why everything works up to 256 pixels, and immediately breaks beyond that.

I'm still figuring out how the code works, so don't know how to fix this yet.