[request] Add system flag to make background fill the window
I'd like to avoid this:

![Screen Shot 2021-12-27 at 12 08 45 PM](https://user-images.githubusercontent.com/6473/147493012-3324f267-69a9-436d-a37d-1e609b778c6e.png)

Could we have a system flag that would cause the HTML background to always match the "screen background color" (color 1) as detailed on https://wasm4.org/docs/guides/basic-drawing.  So that the break between the browser HTML and the canvas wasn't so dead obvious?

```
pub const SYSTEM_BACKGROUND_FILLS_SCREEN: u8 = 4;
```

On native clients where the resolution or window is larger than the actual content size this could also control that background color.