Windows: ScrollContainer loses position when window is resized

**To Reproduce**

* Run the app from [#1604](https://github.com/beeware/toga/issues/1604) on Windows.
* Resize the window so the scroll bar is visible, and scroll down to the bottom.
* Resize the window again. Even the smallest change in the window size causes the scroll position to jump back to the top.

**Expected behavior**

When the window is resized, the scroll position should be retained as closely as possible.

**Environment:**
- Operating System: Windows 10
- Python version: 3.8
- Software versions:
- Briefcase: 560c1aae87
- Toga: 9a292f8e26ffbe

Start with the ScrollContainer implementation and its Windows resize handling, using the app from #1604 to reproduce the behavior. Resize the window after scrolling to the bottom and inspect when the position returns to the top. Done means resizing retains the scroll position as closely as possible without breaking the visible scrollbar.

Written by the indexing model from the issue text.