Fix best size calculation with GTK3

GTK3 reports preferred size of hidden widgets as zero.
see #15896


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75668 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/combocmn.cpp

@@ -1391,7 +1391,6 @@ wxSize wxComboCtrlBase::DoGetSizeFromTextSize(int xlen, int ylen) const
     else
     {
         wxComboBox* cb = new wxComboBox;
-        cb->Hide();
         cb->Create(const_cast<wxComboCtrlBase*>(this), wxID_ANY);
         if ( m_font.IsOk() )
             cb->SetFont(m_font);