Move onscreen rendering into SGCT's render2D function (closes #181)

Same fix makes GUI windows work again (closes #412)

include/openspace/engine/openspaceengine.h

@@ -87,6 +87,7 @@ public:
     void postSynchronizationPreDraw();
     void render(const glm::mat4& sceneMatrix, const glm::mat4& viewMatrix,
         const glm::mat4& projectionMatrix);
+    void drawOverlays();
     void postDraw();
     void keyboardCallback(Key key, KeyModifier mod, KeyAction action);
     void charCallback(unsigned int codepoint, KeyModifier mod);
@@ -133,6 +134,7 @@ public:
         PreSync,         // Callback for the end of the pre-sync function
         PostSyncPreDraw, // Callback for the end of the post-sync-pre-draw function
         Render,          // Callback for the end of the render function
+        Draw2D,          // Callback for the two-dimensional rendering functions
         PostDraw         // Callback for the end of the post-draw function
     };
 
@@ -218,6 +220,7 @@ private:
         std::vector<std::function<void()>> preSync;
         std::vector<std::function<void()>> postSyncPreDraw;
         std::vector<std::function<void()>> render;
+        std::vector<std::function<void()>> draw2D;
         std::vector<std::function<void()>> postDraw;
 
         std::vector<std::function<bool (Key, KeyModifier, KeyAction)>> keyboard;