This fixes#572 to change the backdrop alpha when the view size or
its size class changes.
The main change is that `true` is passed as a
`forceLayout` parameter into `viewWillTransition(to:with:)` callbacks.
Because it's necessary for the backdrop alpha's update when the view
size or its size class changes.
This also fixes a regression at `9c45c31` commit.
```diff
- layoutAdapter.activateLayout(for: state, forceLayout: true)
+ layoutAdapter.activateLayout(for: state, forceLayout: forceLayout)
```
The behavior before the above change indicates that the method has
worked well even when `forceLayout` is set to `true` in their callbacks.
Additional improvements:
* Format `activateLayout(forceLayout:contentInsetAdjustmentBehavior:)`
* Add `_floor` function for `test_updateBackdropAlpha()`
Shin Yamamoto
b941f91556