Fixed parts of toolbar background not being drawn with older comctl32.dll.

When using comctl32.dll versions older than 6.0 toolbar icons would not have their background drawn. Fix this in a rough way by always completely erasing the background like was done before. Regression since r62971.

Closes #13512.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71341 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/toolbar.h

@@ -67,6 +67,7 @@ public:
 
     void OnMouseEvent(wxMouseEvent& event);
     void OnSysColourChanged(wxSysColourChangedEvent& event);
+    void OnEraseBackground(wxEraseEvent& event);
 
     void SetFocus() {}