three.js: consider using json Object/Scene format

[hamogu]

The current three.js code writes actual test files, which are directly executable. It would be much easier to just produce json files in Python and load them into Three.js with the generic ObjectLoader

https://threejs.org/docs/#api/en/loaders/ObjectLoader

[hamogu]

More precisely, this can replace the homegrown json loader in marxs and I can throw out the "write javascript directly" writer. Taken together, three.js should be a format that I can pretty easily output again.