Add a method for creating a unique name for screenspace renderable (closes #652)

include/openspace/rendering/screenspacerenderable.h

@@ -75,6 +75,8 @@ public:
 
 protected:
     void createShaders();
+    std::string makeUniqueIdentifier(std::string name);
+
     glm::mat4 scaleMatrix();
     glm::mat4 globalRotationMatrix();
     glm::mat4 translationMatrix();