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/GLCanvas3DManager.hpp

@@ -91,6 +91,7 @@ public:
     void init_gl();
 
     bool init(wxGLCanvas* canvas);
+    void destroy();
 
     GLCanvas3D* get_canvas(wxGLCanvas* canvas);