Tear down the Design canvas at shutdown

The Design tab's viewport is the fourth GLCanvas3D on the shared GL context and
the only one the plater does not own, so unbind_canvas_event_handlers() and
reset_canvas_volumes() never reached it — the macOS Command+Q and Debian cases
those calls exist for. Its frame-level handlers become members so they can be
unbound.

src/slic3r/GUI/CAD/DesignPanel.hpp

@@ -51,6 +51,8 @@ public:
     explicit DesignPanel(wxWindow* parent);
     void on_tab_shown();        // re-sync bed to the active printer when the Design tab is activated
     void on_tab_hidden();       // another tab took over: take the viewport status line down with us
+    void unbind_canvas_event_handlers();   // app close / language switch, from the plater's teardown
+    void reset_canvas_volumes();
     void clear_document();      // New Project / Open Project: drop the document with the project
     // Rebuild off the UI thread (progress dialog only if it turns out to be slow), so a feature
     // op on a heavy imported solid does not freeze the window. Returns m_doc.recompute()'s result.