…ed as constant

We need to do the ensure_window pass before the binding_analysis
otherwise the with and height property of the root element
will be considered as constant and will not adapt to the window size.

Unfortunately we can't really test that now because our test system
doesn't really test the Window, but here is a testcase:

```
  MainWindow := Rectangle {
     preferred-width: 400px;
     preferred-height: 400px;
     background: yellow;
     VerticalLayout {
          Rectangle {
              background: red;
              Text { text: root.width / 1px; }
          }
     }
  }
```

(we should see the red rectangle)