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

build/msw/wx_core.dsp

@@ -3170,6 +3170,10 @@ SOURCE=..\..\src\generic\numdlgg.cpp
 # End Source File
 # Begin Source File
 
+SOURCE=..\..\src\generic\panelg.cpp
+# End Source File
+# Begin Source File
+
 SOURCE=..\..\src\generic\printps.cpp
 # End Source File
 # Begin Source File