Fix: Scrolling Preferences could cause blank UI

Steps to reproduce were:
* Open Preferences
* Choose "Input" category
* Scroll to the bottom
* Choose "Interface" category

The newly activated category should now use the scrolling set previously
in the other category, causing the contents to be out of view. You
would have to scroll to bring it back.

Now scrolling is stored per category.

source/blender/editors/space_userpref/space_userpref.c

@@ -123,7 +123,7 @@ static void userpref_main_region_init(wmWindowManager *wm, ARegion *ar)
 
 static void userpref_main_region_draw(const bContext *C, ARegion *ar)
 {
-	ED_region_panels(C, ar);
+	ED_region_panels_ex(C, ar, NULL, U.userpref, true);
 }
 
 static void userpref_operatortypes(void)