Sort screenspacerenderables in correct render order

include/openspace/rendering/renderengine.h

@@ -138,6 +138,7 @@ public:
 	// Temporary fade functionality
 	void startFading(int direction, float fadeDuration);
 
+    void sortScreenspaceRenderables();
     // This is temporary until a proper screenspace solution is found ---abock
     struct {
         glm::vec2 _position;

include/openspace/rendering/screenspacerenderable.h

@@ -55,8 +55,11 @@ public:
 	virtual void update() = 0;
 	virtual bool isReady() const = 0;
 	bool isEnabled() const;
+	
 	glm::vec2 euclideanPosition() const {return _euclideanPosition.value();};
 	glm::vec2 sphericalPosition() const {return _sphericalPosition.value();};
+	float depth() const {return _depth.value();};
+	
 	void move(glm::vec2 v){
 		if(_useFlatScreen.value()){
 			glm::vec2 pos = _euclideanPosition.value();