[WPF] Fix loading of 8bit paletted images

WPF requires a palette to be set when loading paletted images. We need
to maintain the BitmapSource palette in ConvertBitmapTo96DPI().

Edit: without this change, calling `Image.FromFile()` on 8bit / paletted bitmaps fails with an InvalidOperationException at [ImageHandler.cs:75](https://github.com/thefiddler/xwt/blob/master/Xwt.WPF/Xwt.WPFBackend/ImageHandler.cs#L75). With this change, both paletted and color bitmaps work as expected.