Add wxPanel::SetBackgroundBitmap().

This method provides a simple way to set a background bitmap without defining
an EVT_ERASE_BACKGROUND handler and, more importantly, one that works
correctly in wxMSW for a window with children as it paints the background of
transparent children too.

Add a test of this method to the erase sample.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67253 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/univ/panel.h

@@ -44,6 +44,13 @@ public:
     )
 #endif // WXWIN_COMPATIBILITY_2_8
 
+protected:
+    virtual void DoSetBackgroundBitmap(const wxBitmap& bmp)
+    {
+        // We have support for background bitmap even at the base class level.
+        SetBackground(bmp, wxALIGN_NOT, wxTILE);
+    }
+
 private:
     wxDECLARE_DYNAMIC_CLASS_NO_COPY(wxPanel);
 };