[WPF] Fix high-DPI image scaling

This should fix issue #401.

Xwt.WPF/Xwt.WPFBackend/ImageHandler.cs

@@ -459,8 +459,15 @@ namespace Xwt.WPFBackend
 
 				var f = GetBestFrame (actx, scaleFactor, idesc.Size.Width, idesc.Size.Height, false);
 				var bmpImage = f as BitmapSource;
-				if (bmpImage != null && (bmpImage.PixelHeight != idesc.Size.Height || bmpImage.PixelWidth != idesc.Size.Width))
-					f = new TransformedBitmap (bmpImage, new ScaleTransform (idesc.Size.Width / bmpImage.PixelWidth, idesc.Size.Height / bmpImage.PixelHeight));
+
+				// When an image is a single bitmap that doesn't have the same intrinsic size as the drawing size, dc.DrawImage makes a very poor job of down/up scaling it.
+				// Thus we handle this manually by using a TransformedBitmap to handle the conversion in a better way when it's needed.
+
+				var scaledWidth = idesc.Size.Height * scaleFactor;
+				var scaledHeight = idesc.Size.Width * scaleFactor;
+				if (bmpImage != null && (Math.Abs (bmpImage.PixelHeight - scaledHeight) > 0.001 || Math.Abs (bmpImage.PixelWidth - scaledWidth) > 0.001))
+					f = new TransformedBitmap (bmpImage, new ScaleTransform (scaledWidth / bmpImage.PixelWidth, scaledHeight / bmpImage.PixelHeight));
+
 				dc.DrawImage (f, new Rect (x, y, idesc.Size.Width, idesc.Size.Height));
 
 				if (idesc.Alpha < 1)