Also:
* Add three explicit sizing modes to Stack for non-positioned
children: loose, expand, and passthrough. (All three are used.)
* Fix a bug whereby layers would try to paint in the same frame as
they were removed from layout (but not detached).
* Fix a bug whereby Offstage wasn't properly marking the parent dirty
when changing its sizedByParent flag.
* Explicitly make Overlay expand non-positioned children.
* Explicitly have InputDecoration pass through the constraints from
its Row to its Stack children.
