start actually implementing multi-window feature, still rough around the edges

fix crash when closing main window if sub windows are involved

fix OpenGL context handling, still need to fix when changing display type

src/frontend/qt_sdl/Screen.h

@@ -197,6 +197,7 @@ private:
     void setupScreenLayout() override;
 
     std::unique_ptr<GL::Context> glContext;
+    bool glInited;
 
     GLuint screenVertexBuffer, screenVertexArray;
     GLuint screenTexture;