Added "scene" property to simple test models

[javagl]

According to the specification, the runtime is not required
to render anything when the scene property is undefined.

Fixes #240

[emackey]

Also fixes #166 I believe.

[javagl]

@emackey In some way, yes, but 166 also considered adding an example with multiple scenes. (I think there is no sample right now with multiple scenes, so I'm currently creating one - as usual for the "simple" test models, it's a trivial one, just to test which viewers choke on that (and in which way...))

[emackey]

@javagl Actually I'm not sure the WG wants an official sample until we know what the plan is for multiple scenes. There's an issue open for considering deprecating multiple scenes. KhronosGroup/glTF#1542

[javagl]

@emackey Thanks for that hint. I also just had a look at the issue, but have not yet read through the discussion there - it seems like this is targeting glTF-next, and so one might consider adding the sample anyhow. But in doubt, for now, it resides in a branch at https://github.com/javagl/glTF-Sample-Models/tree/MultipleScenes , and 166 can probably be closed for now.