Revert "Revert "[WPF] Fix high-DPI image scaling""

This reverts commit 67a15a9.

The change introduced in commit db94973
is required for proper HiDPI support. But the scaled sizes are
swapped which causes the regression mentioned in
67a15a9. Fix follows.

Xwt.WPF/Xwt.WPFBackend/ImageHandler.cs

@@ -459,8 +459,15 @@ public void Draw (ApplicationContext actx, SWM.DrawingContext dc, double scaleFa
 
 				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)