Fix more transparency issues

Author: DSPaul

src/ViewModels/Sources/PdfSourceViewModel.cs

@@ -94,6 +94,10 @@ public override async Task<bool> FetchCover(Codex codex)
                     await image.ReadAsync(codex.Sources.Path, ReadSettings);
                     image.Format = MagickFormat.Png;
 
+                    //some pdf's are transparent, expecting a white page underneath
+                    image.BackgroundColor = new MagickColor("#FFFFFF");
+                    image.Alpha(AlphaOption.Remove);
+
                     await image.WriteAsync(codex.CoverArt);
                     CoverService.CreateThumbnail(codex, image);
                 }