Support the unpackaged format for font loading (#20790)

src/Core/src/ImageSources/FontImageSourceService/FontImageSourceService.Windows.cs

@@ -6,8 +6,10 @@
 using Microsoft.Graphics.Canvas;
 using Microsoft.Graphics.Canvas.Text;
 using Microsoft.Graphics.Canvas.UI.Xaml;
+using Microsoft.Maui.ApplicationModel;
 using Microsoft.Maui.Devices;
 using Microsoft.Maui.Graphics;
+using Microsoft.Maui.Storage;
 using WImageSource = Microsoft.UI.Xaml.Media.ImageSource;
 
 namespace Microsoft.Maui
@@ -122,6 +124,18 @@ string GetFontSource(IFontImageSource imageSource)
 				}
 			}
 
+			// unpackaged apps can't load files using packaged schemes
+			if (!AppInfoUtils.IsPackagedApp)
+			{
+				var fontUri = new Uri(fontSource, UriKind.RelativeOrAbsolute);
+
+				var path = fontUri.AbsolutePath.TrimStart('/');
+				if (FileSystemUtils.TryGetAppPackageFileUri(path, out var uri))
+				{
+					fontSource = uri + fontUri.Fragment;
+				}
+			}
+
 			return fontSource;
 		}
 	}