added MSWAlwaysDrawBg() to account for controls such as sliders which must not draw background with transparent brush no matter what

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@30934 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/control.cpp

@@ -371,7 +371,7 @@ WXHBRUSH wxControl::MSWControlColor(WXHDC pDC)
     }
 #endif // wxUSE_UXTHEME
 
-    return ::GetStockObject(NULL_BRUSH);
+    return MSWAlwaysDrawBg() ? 0 : ::GetStockObject(NULL_BRUSH);
 }
 
 WXHBRUSH wxControl::MSWControlColorDisabled(WXHDC pDC)