Separate Renderable::initialize into initialize and initializeGL functions

Handle initialize in a separate thread and display rudimentary loading screen

include/openspace/engine/wrapper/sgctwindowwrapper.h

@@ -79,6 +79,8 @@ public:
 
     void takeScreenshot(bool applyWarping = false) const override;
 
+    void swapBuffer() const override;
+
 private:
     properties::FloatProperty _eyeSeparation;
     properties::BoolProperty _showStatsGraph;

include/openspace/engine/wrapper/windowwrapper.h

@@ -270,6 +270,8 @@ public:
      */
     virtual void takeScreenshot(bool applyWarping = false) const;
 
+    virtual void swapBuffer() const;
+
     struct WindowWrapperException : public ghoul::RuntimeError {
         explicit WindowWrapperException(const std::string& msg);
     };