SceneGraph: remove constructor signature requirements in addChild().

It now calls setParent() afterwards instead of requiring the last
constructor parameter to be a parent pointer.

doc/changelog.dox

@@ -137,6 +137,12 @@ See also:
     @ref Corrade::Containers::ArrayView are now removed. This should have a
     significant positive effect on compile times of code using the @ref GL,
     @ref Audio, @ref Trade and @ref Text librariess
+-   @ref SceneGraph::Object:addChild() no longer requires the type constructor
+    to have the last parameter a parent object object pointer, as that was
+    quite limiting. Instead it's calling @ref SceneGraph::Object::setParent()
+    afterwards. This can cause compilation breakages in case the type
+    constructor has the parent parameter non-optional, pass the parent
+    explicitly in that case.
 
 @section changelog-2020-06 2020.06
 

src/Magnum/SceneGraph/Object.h

@@ -195,10 +195,13 @@ template<class Transformation> class Object: public AbstractObject<Transformatio
          * @brief Add a child
          *
          * Calling `object.addChild<MyObject>(args...)` is equivalent to
-         * `new MyObject{args..., &object}`.
+         * `new MyObject{args...}` followed by an appropriate @ref setParent()
+         * call.
          */
         template<class T, class ...Args> T& addChild(Args&&... args) {
-            return *(new T{std::forward<Args>(args)..., this});
+            T* child = new T{std::forward<Args>(args)...};
+            child->setParent(this);
+            return *child;
         }
 
         /**