Add Optix render window

**Original report ([archived issue](https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-rendering/issues/18)) by Ian Chen (Bitbucket: [Ian Chen](https://bitbucket.org/%7Beaa6fca5-6deb-43f6-907f-971c144735dd%7D/), GitHub: [iche033](https://github.com/iche033)).**

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The `RenderWindow` class works by attaching to an existing GUI window using its ID / window [?at=default&fileviewer=file-view-default#RenderTarget.hh-105 handle string ](https://github.com/ignitionrobotics/ign-rendering/blob/9a899e646abf5e093fe9f74710e2d3b9f5719950/include/ignition/rendering/RenderTarget.hh). This currently works for ogre as it already has built-in [?at=default&fileviewer=file-view-default render window support ](https://github.com/ignitionrobotics/ign-rendering/blob/9a899e646abf5e093fe9f74710e2d3b9f5719950/src/ogre/OgreRenderTarget.cc). The [?at=default&fileviewer=file-view-default OptixRenderWindow ](https://github.com/ignitionrobotics/ign-rendering/blob/9a899e646abf5e093fe9f74710e2d3b9f5719950/src/optix/OptixRenderTarget.cc) class is currently just a stub and doesn't do anything.

For optix, we currently require the user to save the image data a buffer in memory then display it into their own window (e.g. see [?at=default&fileviewer=file-view-default#GlutWindow.cc-95 this line in simple_demo that captures data into image data object ](https://github.com/ignitionrobotics/ign-rendering/blob/9a899e646abf5e093fe9f74710e2d3b9f5719950/examples/simple!UNITO-UNDERSCORE!demo/GlutWindow.cc) example) which requires users to write more code.

It would be nice to have a common `RenderWindow` API that masks these differences.