[SizerBase] GridSplitter ResizeDirection doesn't update thumb/cursor until next interaction

### Describe the bug

Was noticing an issue where in a VSM I update the ResizeDirection of a GridSplitter between Rows to Columns (it's a splitter that can change between a horizontal doc split and a vertical one, see: XAML Studio).

However, the thumb/cursor for the Grid Splitter doesn't change from the horizontal mode to the vertical one until after the control is interacted with again.

If I also change the Orientation value, it works as expected. But since the ResizeDirection updates the Orientation value, I figured I only needed one.

### Steps to reproduce

```text
1. Create a Grid with 3 rows and 3 columns
2. Put splitter in middle
3. Swap it's ResizeDirection

(This would probably just make a good sample, I should just copy over what I have to one as another good example.)
```

### Expected behavior

Sizer updates with single changed property.

### Screenshots

_No response_

### Code Platform

- [x] UWP
- [ ] WinAppSDK / WinUI 3
- [ ] Web Assembly (WASM)
- [ ] Android
- [ ] iOS
- [ ] MacOS
- [ ] Linux / GTK

### Windows Build Number

- [ ] Windows 10 1809 (Build 17763)
- [ ] Windows 10 1903 (Build 18362)
- [ ] Windows 10 1909 (Build 18363)
- [ ] Windows 10 2004 (Build 19041)
- [ ] Windows 10 20H2 (Build 19042)
- [ ] Windows 10 21H1 (Build 19043)
- [ ] Windows 10 21H2 (Build 19044)
- [ ] Windows 10 22H2 (Build 19045)
- [ ] Windows 11 21H2 (Build 22000)
- [ ] Other (specify)

### Other Windows Build number

_No response_

### App minimum and target SDK version

- [ ] Windows 10, version 1809 (Build 17763)
- [ ] Windows 10, version 1903 (Build 18362)
- [ ] Windows 10, version 1909 (Build 18363)
- [ ] Windows 10, version 2004 (Build 19041)
- [ ] Windows 10, version 2104 (Build 20348)
- [ ] Windows 11, version 22H2 (Build 22000)
- [ ] Other (specify)

### Other SDK version

_No response_

### Visual Studio Version

_No response_

### Visual Studio Build Number

_No response_

### Device form factor

_No response_

### Additional context

_No response_

### Help us help you

Yes, I'd like to be assigned to work on this item.

Beginne mit der SizerBase- und GridSplitter-Implementierung für das UWP-Steuerelement und reproduziere dann das Problem, indem du ResizeDirection von Rows in Columns änderst, ohne mit dem Splitter zu interagieren. Die Aufgabe ist erledigt, wenn Thumb und Cursor unmittelbar nach der einmaligen Änderung der Eigenschaft aktualisiert werden, ohne dass eine Änderung von Orientation oder eine zweite Interaktion erforderlich ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.