Added member context to GafferScene::InteractiveRender

GafferScene::InteractiveRender now has a context member variable, which is used while it's evaluating the scene. It's also got accessor methods, and is automatically set to the context of the script it belongs to (if one exists)
Merge pull request #455 from davidsminor/interactiveRenderContext

Added member context to GafferScene::InteractiveRender