Render to external window

How to render to external window? I was expecting if I set the `winID` to the QWidget window id that it would draw the content to this location. This works if I use ogre2 directly so not sure why this does not work here.

```
std::map params;
params["winID"] = std::to_string(winId());
auto* engine = ignition::rendering::engine("ogre2", params);
```

Start at the `ignition::rendering::engine("ogre2", params)` entry point and trace how the `winID` parameter is handled for the OGRE2 engine. Compare that path with the direct OGRE2 integration described in the issue; done means rendering appears in the QWidget identified by `winId()`.

Written by the indexing model from the issue text.