SplashScreen areas with transparency turn black when fading out in .NET 10

### Description

The `SplashScreen` class used to display an image on application startup supports PNG files with transparency. While the screen is present, the transparency works as expected and you can see through these areas. The moment the image starts fading out, the transparency is lost and all transparent areas turn black.

### Reproduction Steps

Create a new WPF app, add a PNG with transparency to the solution (eg the dice one from the wikipedia PNG page) and set its build action to `Resource`. Add the following code to the `App` constructor:
`new SplashScreen("PNG_transparency_demonstration_1.png").Show(true, true);`
Run the application, and observe how the image appears correctly initially but then acquires a black background as it fades out.

### Expected behavior

The transparency is maintained while fading out

### Actual behavior

The background turns black

### Regression?

Yes, I've upgraded a .NET 6 project built using VS 2022 to .NET 10 using VS 2026 and have now encountered this. This is a new computer, so I don't have VS 2022 or any older .NET versions installed to help narrow down where it broke.

### Known Workarounds

_No response_

### Impact

Jarring visual experience as the splash screen fades out

### Configuration

.NET 10, Windows 11 Pro 25H2, both x86 and x64. This is the only configuration I've tried it on.

### Other information

_No response_