Add missing references to wxSize parameters in wxBitmapBundle

Pass wxSize by const reference instead of "const" value.

Note that passing wxSize by value might be not such a bad thing,
actually, but we use const reference for it everywhere else, so do it
here as well for consistency (and the original intention was to do it
like this, missing "&" was just a typo subsequently propagated through
copy-pasting).

No real changes.

samples/toolbar/toolbar.cpp

@@ -520,7 +520,7 @@ void MyFrame::PopulateToolbar(wxToolBarBase* toolBar)
                     return m_sizeDef;
                 }
 
-                wxBitmap GetBitmap(const wxSize size) wxOVERRIDE
+                wxBitmap GetBitmap(const wxSize& size) wxOVERRIDE
                 {
                     // In this simple implementation we don't bother caching
                     // anything.