Hopefully a workaround for crashes and hang ups at closing on Windows.

Release the OpenGL contexts not by a destructor of a static class,
but explicitely (predictively).

src/slic3r/GUI/3DScene.hpp

@@ -647,6 +647,7 @@ public:
     static void remove_all_canvases();
 
     static bool init(wxGLCanvas* canvas);
+    static void destroy();
 
     static GUI::GLCanvas3D* get_canvas(wxGLCanvas* canvas);