Open Radar

 
When rendering a ZStack, if any Views with a lower `zIndex` than its siblings applies an `onHover` modifier, that closure gets called even if the View is overlayed (and potentially hidden) by other Views.

Steps to reproduce:
This is enough to exemplify the behavior:
```
ZStack {
    Rectangle()
        .fill(Color.red)
        .onHover { active in
            if active {
                fatalError("Shouldn't be hoverable [\(active)]")
            }
        }

    Rectangle()
        .fill(Color.white)
}
```

What did you expect to happen?
The `onHover` closure shouldn't have been called (specially receiving `true` in the argument) since the view is being covered and, therefore, not actually being hovered.

What actually happened?
The closure still get's called and receives `true` and the mouse pointer enters the view's bounds.