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

interface/wx/panel.h

@@ -124,6 +124,21 @@ public:
     */
     void OnSysColourChanged(wxSysColourChangedEvent& event);
 
+    /**
+        Set the background bitmap for this panel.
+
+        If @a bmp is a valid bitmap, this bitmap will be tiled over the panel
+        background and show through any of its transparent children. Passing an
+        invalid bitmap reverts to the default background appearance.
+
+        Notice that you must not prevent the base class EVT_ERASE_BACKGROUND
+        handler from running (i.e. not to handle this event yourself) for this
+        to work.
+
+        @since 2.9.2
+    */
+    void SetBackgroundBitmap(const wxBitmap& bmp);
+
     /**
         Overrides wxWindow::SetFocus().