Fix error about duplicate PNG image handler in the drawing sample

Add the handler earlier, as creating the handler main frame uses
wxArtProvider::GetBitmap() which can add the PNG handler on some
platforms (e.g. macOS, when using wxTangoArtProvider), so calling
wxImage::AddHandler() again after creating the frame could result in a
debug error message about adding the same handler twice.

samples/drawing/drawing.cpp

@@ -461,6 +461,10 @@ bool MyApp::OnInit()
     if ( !wxApp::OnInit() )
         return false;
 
+#if wxUSE_LIBPNG
+      wxImage::AddHandler( new wxPNGHandler );
+#endif
+
     // Create the main application window
     MyFrame *frame = new MyFrame("Drawing sample",
                                  wxDefaultPosition, wxSize(550, 840));
@@ -477,9 +481,6 @@ bool MyApp::OnInit()
         // still continue, the sample can be used without images too if they're
         // missing for whatever reason
     }
-#if wxUSE_LIBPNG
-      wxImage::AddHandler( new wxPNGHandler );
-#endif
 
     return true;
 }