Impeller: Fix nested clipping in Aiks

Applying a single clip path works, but applying multiple results in no clipping:

```cpp
canvas.ClipPath(PathBuilder{}.AddCircle({200, 400}, 300).TakePath());
canvas.ClipPath(PathBuilder{}.AddCircle({600, 400}, 300).TakePath());
canvas.DrawRect(Rect::MakeXYWH(200, 200, 400, 400), paint);
```