Fix initial wxRadioBox buttons positions in wxMSW.

We only updated the button positions when the radio box was moved or resized
after being created but didn't do it initially, so a radio box created with
fixed position and size didn't lay out its buttons correctly. Do lay them out
immediately after creating the radio box to fix this.

Closes #13912.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70498 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/radiobox.h

@@ -149,6 +149,9 @@ protected:
     // get the total size occupied by the radio box buttons
     wxSize GetTotalButtonSize(const wxSize& sizeBtn) const;
 
+    // Adjust all the buttons to the new window size.
+    void PositionAllButtons(int x, int y, int width, int height);
+
     virtual void DoSetSize(int x, int y,
                            int width, int height,
                            int sizeFlags = wxSIZE_AUTO);