Mica get's overdrawn by Swapchain when Swapchain covers entire screen

Nobody has claimed this yet.

### Describe the bug

Consider the following scenario where we create a CanvasSwapchainPanel from Win2D while the Window has Mica as backdrop:

```xaml

```

```c#
public sealed partial class MainWindow : Window
{
public MainWindow()
{
this.InitializeComponent();
}

CanvasSwapChain csc;

private void Swappy_SizeChanged(object sender, SizeChangedEventArgs e)
{
this.csc = new CanvasSwapChain(CanvasDevice.GetSharedDevice(), Math.Max((float)Swappy.ActualWidth, 1), Math.Max((float)Swappy.ActualHeight, 1), 96);
this.Swappy.SwapChain = this.csc;
DrawInPanel();
}

private void DrawInPanel()
{
using (var ds = this.csc.CreateDrawingSession(Microsoft.UI.Colors.Transparent))
{
ds.Clear(Microsoft.UI.Colors.Transparent);
ds.DrawText("Some text", 0, 0, Microsoft.UI.Colors.Red);
ds.DrawRectangle(0, 0, 100, 100, Microsoft.UI.Colors.Red);
}
this.csc.Present(0);
}
}
```

With this, the output looks like this:

However if we change the Grid or the CanvasSwapChainPanel to have a margin or padding of 1 in any of the directions, like this:
```XAML
[...]

[...]
```
Mica works:

To add to the unusualness, when one sets the margin and padding of the Grid and CSCP to cancel out like this:

```



```

Mica doesn't work anymore:

Note that sometimes, the screen only turns black after resizing the window.

### Steps to reproduce the bug
Repro project: [Mica repro.zip](https://github.com/user-attachments/files/17812056/Mica.repro.zip)
1. Create a project and add Win2D as dependency
2. Add the following XAML:
```xaml

```
3. Add the following C#
```c#
public sealed partial class MainWindow : Window
{
public MainWindow()
{
this.InitializeComponent();
}

CanvasSwapChain csc;

private void Swappy_SizeChanged(object sender, SizeChangedEventArgs e)
{
this.csc = new CanvasSwapChain(CanvasDevice.GetSharedDevice(), Math.Max((float)Swappy.ActualWidth, 1), Math.Max((float)Swappy.ActualHeight, 1), 96);
this.Swappy.SwapChain = this.csc;
DrawInPanel();
}

private void DrawInPanel()
{
using (var ds = this.csc.CreateDrawingSession(Microsoft.UI.Colors.Transparent))
{
ds.Clear(Microsoft.UI.Colors.Transparent);
ds.DrawText("Some text", 0, 0, Microsoft.UI.Colors.Red);
ds.DrawRectangle(0, 0, 100, 100, Microsoft.UI.Colors.Red);
}
this.csc.Present(0);
}
}
```
4. Observe Mica not working

### Expected behavior

Mica should work without having to set a margin somewhere.

### Screenshots

_No response_

### NuGet package version

WinUI 3 - Windows App SDK 1.6.3: 1.6.241114003

### Windows version

_No response_

### Additional context

Windows version 24H2: 26100.2314

Run the attached Mica repro with the MainWindow XAML and C# code on Windows 24H2 with Windows App SDK 1.6.3. Start at Swappy_SizeChanged, CanvasSwapChainPanel, and MicaBackdrop, comparing full-screen coverage with a one-pixel margin and resizing. Done means Mica remains visible without a margin and after resizing.

Written by the indexing model from the issue text.