Account for scrolling when setting the background brush origin in wxMSW.

We must use physical coordinates for the brush origin to account for the
coordinates offset in scrolled windows, so add MSWAdjustBrushOrg() and call it
from MSWGetBgBrushForChild().

Closes #14917.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73495 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/scrolwin.h

@@ -399,13 +399,19 @@ public:
 #endif
     }
 
+#ifdef __WXMSW__
     // we need to return a special WM_GETDLGCODE value to process just the
     // arrows but let the other navigation characters through
-#ifdef __WXMSW__
     virtual WXLRESULT MSWWindowProc(WXUINT nMsg, WXWPARAM wParam, WXLPARAM lParam)
     {
         return FilterMSWWindowProc(nMsg, T::MSWWindowProc(nMsg, wParam, lParam));
     }
+
+    // Take into account the scroll origin.
+    virtual void MSWAdjustBrushOrg(int* xOrg, int* yOrg) const
+    {
+        CalcUnscrolledPosition(*xOrg, *yOrg, xOrg, yOrg);
+    }
 #endif // __WXMSW__
 
     WX_FORWARD_TO_SCROLL_HELPER()