Ownership questions

**Original report ([archived issue](https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-rendering/issues/13)) by Louise Poubel (Bitbucket: [chapulina](https://bitbucket.org/%7B5cfa2075-477b-4ded-bdb9-8d2479544ec4%7D/), GitHub: [chapulina](https://github.com/chapulina)).**

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I noticed on the examples that it is common to use `Scene::Create*` functions to instantiate most of the rendering types, as opposed to directly calling the type's constructor. All `Create*` functions return shared pointers.

1. Does the scene keep shared ownership of everything which is created by `Scene::Create*`?

```
This seems to be true for some types but not others, for example:
```

```
auto light = scene->CreateDirectionalLight();
ASSERT_EQ(light.use_count(), 2u); // passes
```

```
auto ray = scene->CreateRayQuery();
ASSERT_EQ(ray.use_count(), 1u); // passes
```

I'd suggest we return `unique_ptr`s for those which the scene won't be keeping1. When calling `Visual::AddChild`, is the ownership shared with the visual? The same goes for other `Add` and `Set` functions which take `shared_ptr`s

1. What's the appropriate way to destroy something which was created with `Scene::Create*`? There are some `Scene::Destroy*` functions, but there isn't a 1-1 relation to the creates. How to destroy the missing ones?

1. Is there a situation when an object should / could be instantiated without the use of `Scene::Create*`?

Start by tracing the ownership behavior of the Scene::Create* and Scene::Destroy* functions, then inspect Visual::AddChild and the other Add/Set methods that accept shared pointers. Compare those paths with the light and ray-query examples; done requires an agreed, documented ownership and destruction policy before any API changes can be scoped.

Written by the indexing model from the issue text.