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

src/msw/panel.cpp

@@ -24,6 +24,7 @@
 #endif
 
 #ifndef WX_PRECOMP
+    #include "wx/brush.h"
     #include "wx/panel.h"
 #endif // WX_PRECOMP
 
@@ -44,3 +45,17 @@ bool wxPanel::HasTransparentBackground()
 
     return false;
 }
+
+void wxPanel::DoSetBackgroundBitmap(const wxBitmap& bmp)
+{
+    delete m_backgroundBrush;
+    m_backgroundBrush = bmp.IsOk() ? new wxBrush(bmp) : NULL;
+}
+
+WXHBRUSH wxPanel::MSWGetCustomBgBrush()
+{
+    if ( m_backgroundBrush )
+        return (WXHBRUSH)m_backgroundBrush->GetResourceHandle();
+
+    return wxPanelBase::MSWGetCustomBgBrush();
+}