SceneGraph: ability to access parent from AbstractObject.

Didn't do any tests because there's nothing for AbstractObject anyway
(it's on my TODO list, though).

src/Magnum/SceneGraph/AbstractObject.h

@@ -170,6 +170,12 @@ template<UnsignedInt dimensions, class T> class AbstractObject
         /** @overload */
         const AbstractObject<dimensions, T>* scene() const { return doScene(); }
 
+        /** @brief Parent object or `nullptr`, if this is root object */
+        AbstractObject<dimensions, T>* parent() { return doParent(); }
+
+        /** @overload */
+        const AbstractObject<dimensions, T>* parent() const { return doParent(); }
+
         /** @{ @name Object transformation */
 
         /**
@@ -269,6 +275,9 @@ template<UnsignedInt dimensions, class T> class AbstractObject
         virtual AbstractObject<dimensions, T>* doScene() = 0;
         virtual const AbstractObject<dimensions, T>* doScene() const = 0;
 
+        virtual AbstractObject<dimensions, T>* doParent() = 0;
+        virtual const AbstractObject<dimensions, T>* doParent() const = 0;
+
         virtual MatrixType doTransformationMatrix() const = 0;
         virtual MatrixType doAbsoluteTransformationMatrix() const = 0;
         virtual std::vector<MatrixType> doTransformationMatrices(const std::vector<std::reference_wrapper<AbstractObject<dimensions, T>>>& objects, const MatrixType& initialTransformationMatrix) const = 0;

src/Magnum/SceneGraph/Object.h

@@ -363,6 +363,9 @@ template<class Transformation> class Object: public AbstractObject<Transformatio
         Object<Transformation>* doScene() override final;
         const Object<Transformation>* doScene() const override final;
 
+        Object<Transformation>* doParent() override final;
+        const Object<Transformation>* doParent() const override final;
+
         MatrixType MAGNUM_SCENEGRAPH_LOCAL doTransformationMatrix() const override final {
             return transformationMatrix();
         }

src/Magnum/SceneGraph/Object.hpp

@@ -69,6 +69,14 @@ template<class Transformation> const Object<Transformation>* Object<Transformati
     return scene();
 }
 
+template<class Transformation> Object<Transformation>* Object<Transformation>::doParent() {
+    return parent();
+}
+
+template<class Transformation> const Object<Transformation>* Object<Transformation>::doParent() const {
+    return parent();
+}
+
 template<class Transformation> Object<Transformation>& Object<Transformation>::setParent(Object<Transformation>* parent) {
     /* Skip if parent is already parent or this is scene (which cannot have parent) */
     /** @todo Assert for setting parent to scene */