Convert brushes to immutable brushes in FormattedTextImpl.

They need to be accessed from the render thread.

src/Skia/Avalonia.Skia/FormattedTextImpl.cs

@@ -616,6 +616,7 @@ namespace Avalonia.Skia
 
             if (brush != null)
             {
+                brush = ((IMutableBrush)brush)?.ToImmutable() ?? brush;
                 _foregroundBrushes.Insert(0, new KeyValuePair<FBrushRange, IBrush>(key, brush));
             }
         }