[Bug] Oval
I begun implement new feature but see that we have a lot of bug in oval

![wasm4-screenshot](https://user-images.githubusercontent.com/12572222/134766887-9fe7ad8a-468c-4192-b369-36b1a97aabd9.png)

```
import * as w4 from "./wasm4";

let time:  f32 = 0.0;

export function update (): void {
    time += 0.01;

    store<u16>(w4.DRAW_COLORS, 2);

    const sx = u32((1. + Mathf.sin(time)) * 50);
    const sy = u32((1. + Mathf.sin(time)) * 50);

    w4.text ("New Rect", 4, 4);
    w4.rect (20, 20, 31, 31);

    w4.text ("New Oval", 84, 4);
    w4.oval (80 + 20, 20, 31, 31);

}
```

Follow: #13 