HWND coexistence in WinUI 3 XAML with undocumented Microsoft.UI.Composition interfaces

This is an attempt of the POC per this discussion thread and this issue using undocumented Microsoft.UI.Composition interfaces.

Known Limitation

Need to manually handle input hit testing, DPI changes and other input and windowing primitives.
Need to manually handle D2D and DComp device lost events.
Certain XAML control may have rendering issues.
The provided code might have some reference count leak that I haven't got time to address. After all it's a POC.

MIT License