fix: downloading in multi-pages

fix: cannot export video in some place

Pixiview/Illust/ViewIllustPage.xaml.cs

@@ -382,7 +382,7 @@ namespace Pixiview.Illust
             }
 
             var item = items[index];
-            if (index > 0)
+            if (index > 0 && !force)
             {
                 if (item.Loading || item.Image != null)
                 {