SceneGraph: add Drawable::drawables() function.

Similar to Animable::animables().

src/SceneGraph/Drawable.h

@@ -129,6 +129,20 @@ template<UnsignedInt dimensions, class T> class Drawable: public AbstractGrouped
          */
         explicit Drawable(AbstractObject<dimensions, T>& object, DrawableGroup<dimensions, T>* drawables = nullptr): AbstractGroupedFeature<dimensions, Drawable<dimensions, T>, T>(object, drawables) {}
 
+        /**
+         * @brief Group containing this drawable
+         *
+         * If the drawable doesn't belong to any group, returns `nullptr`.
+         */
+        DrawableGroup<dimensions, T>* drawables() {
+            return AbstractGroupedFeature<dimensions, Drawable<dimensions, T>, T>::group();
+        }
+
+        /** @overload */
+        const DrawableGroup<dimensions, T>* drawables() const {
+            return AbstractGroupedFeature<dimensions, Drawable<dimensions, T>, T>::group();
+        }
+
         /**
          * @brief Draw the object using given camera
          * @param transformationMatrix      %Object transformation relative