Force all Renderables to declare an 'isReady' function to circumvent assertion triggers when a crucial element of initialization fails

Fix velocity in RenderableStars

src/rendering/renderable.cpp

@@ -118,4 +118,8 @@ bool Renderable::isVisible() const {
 	return _enabled;
 }
 
+bool Renderable::isReady() const {
+	return true;
+}
+
 }  // namespace openspace