Avoid invalidating the best size in wxStaticBitmap unnecessarily

This is just a micro-optimization: there is no need to call
InvalidateBestSize() if the size of the bitmap doesn't actually change
(as will most often be the case when this method is called after the
control creation).

src/msw/statbmp.cpp

@@ -287,7 +287,9 @@ void wxStaticBitmap::SetImageNoCopy( wxGDIImage* image)
         sizeNew = image->GetSize();
 
     Free();
-    InvalidateBestSize();
+
+    if ( sizeNew != sizeOld )
+        InvalidateBestSize();
 
     m_isIcon = image->IsKindOf( wxCLASSINFO(wxIcon) );
     // the image has already been copied