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/panel.h

@@ -52,6 +52,18 @@ public:
                 const wxString& name = wxPanelNameStr);
 
 
+    // Use the given bitmap to tile the background of this panel. This bitmap
+    // will show through any transparent children.
+    //
+    // 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.
+    void SetBackgroundBitmap(const wxBitmap& bmp)
+    {
+        DoSetBackgroundBitmap(bmp);
+    }
+
+
     // implementation from now on
     // --------------------------
 
@@ -59,6 +71,9 @@ public:
 
     WX_DECLARE_CONTROL_CONTAINER();
 
+protected:
+    virtual void DoSetBackgroundBitmap(const wxBitmap& bmp) = 0;
+
 private:
     wxDECLARE_EVENT_TABLE();
 
@@ -70,6 +85,7 @@ private:
 #elif defined(__WXMSW__)
     #include "wx/msw/panel.h"
 #else
+    #define wxHAS_GENERIC_PANEL
     #include "wx/generic/panelg.h"
 #endif