Avoid using wxDynamicCast() with wxComboCtrl-derived object

In wxUniv wxComboBox does derive from wxComboCtrl, but its wxRTTI
doesn't use wxComboCtrl as the base class for consistency with the other
ports, meaning that wxDynamicCast() to wxComboCtrl fails.

Avoid this problem by storing wxComboCtrl in wxComboPopupWindow, as then
we can just use it later without any casts -- at the price of storing an
extra pointer in a transient object, i.e. without any real cost.

See https://github.com/wxWidgets/wxWidgets/pull/2418

src/common/combocmn.cpp

@@ -339,6 +339,7 @@ public:
                                                 wxSize(20,20),
                                                 style)
     #endif
+                       , m_combo(parent)
     {
         m_inShow = 0;
     }
@@ -351,7 +352,13 @@ protected:
 #endif
 
 private:
+    // This is the same as our parent, but has the right type, so that we can
+    // avoid using casts later.
+    wxComboCtrlBase* const m_combo;
+
     wxByte      m_inShow;
+
+    wxDECLARE_NO_COPY_CLASS(wxComboPopupWindow);
 };
 
 
@@ -391,11 +398,7 @@ bool wxComboPopupWindow::ProcessLeftDown(wxMouseEvent& event)
 // First thing that happens when a transient popup closes is that this method gets called.
 void wxComboPopupWindow::OnDismiss()
 {
-    wxComboCtrlBase* combo = (wxComboCtrlBase*) GetParent();
-    wxASSERT_MSG( wxDynamicCast(combo, wxComboCtrlBase),
-                  wxT("parent might not be wxComboCtrl, but check wxIMPLEMENT_DYNAMIC_CLASS2() macro for correctness") );
-
-    combo->OnPopupDismiss(true);
+    m_combo->OnPopupDismiss(true);
 }
 #endif // wxUSE_POPUPWIN