Set wxProgressDialog::m_parent too

This, in addition to the previous commit, allows Centre() to work
correctly as it needs to retrieve the parent window rectangle.

See #17768.

include/wx/generic/progdlgg.h

@@ -124,8 +124,8 @@ protected:
     // the dialog was shown
     void ReenableOtherWindows();
 
-    // Set the top level parent we store from the parent window provided when
-    // creating the dialog.
+    // Store the parent window as wxWindow::m_parent and also set the top level
+    // parent reference we store in this class itself.
     void SetTopParent(wxWindow* parent);
 
     // return the top level parent window of this dialog (may be NULL)