Fix PopupBox transforming to device coordinates with global system DPI (#3641)

PopupBox correctly transforms from device coordinates using the
PresentationSource transform matrix, but then uses the global system DPI to
transform back to device coordinates. This causes placement issues on systems
with multiple DPI/scaling settings.

Author: Nextra

src/MaterialDesignThemes.Wpf/PopupBox.cs

@@ -617,8 +617,8 @@ private CustomPopupPlacement[] GetPopupPlacement(Size popupSize, Size targetSize
                 throw new ArgumentOutOfRangeException();
         }
 
-        double xTransformed = DpiHelper.TransformToDeviceX(x);
-        double yTransformed = DpiHelper.TransformToDeviceY(y);
+        double xTransformed = DpiHelper.TransformToDeviceX(this, x);
+        double yTransformed = DpiHelper.TransformToDeviceY(this, y);
 
         _popupPointFromLastRequest = new Point(xTransformed, yTransformed);
         return new[] { new CustomPopupPlacement(_popupPointFromLastRequest, PopupPrimaryAxis.Horizontal) };