`rect` does not handle negative offsets properly
Drawing a 5 high full-width rect offscreen at -10 `y`:

```
export fn update() void {
    w4.DRAW_COLORS.* = 0x2222;
    w4.rect(0,-10,160,5);
}
```

**Expectation:**

You do not see any of the rectangle.

**Actually:**

You always see the very bottom 1px of the rect.

<img width="440" alt="Screen Shot 2021-12-25 at 10 31 04 AM" src="https://user-images.githubusercontent.com/6473/147388379-35ce6b4f-b744-4d46-b5cd-ef20755af5f7.png">

