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/generic/panelg.h

@@ -12,6 +12,8 @@
 #ifndef _WX_GENERIC_PANELG_H_
 #define _WX_GENERIC_PANELG_H_
 
+#include "wx/bitmap.h"
+
 class WXDLLIMPEXP_CORE wxPanel : public wxPanelBase
 {
 public:
@@ -40,7 +42,18 @@ public:
     )
 #endif // WXWIN_COMPATIBILITY_2_8
 
+protected:
+    virtual void DoSetBackgroundBitmap(const wxBitmap& bmp);
+
 private:
+    // Event handler for erasing the background which is only used when we have
+    // a valid background bitmap.
+    void OnEraseBackground(wxEraseEvent& event);
+
+
+    // The bitmap used for painting the background if valid.
+    wxBitmap m_bitmapBg;
+
     wxDECLARE_DYNAMIC_CLASS_NO_COPY(wxPanel);
 };