Headless node.js example app

[UX3D-becher]

Pull request #286 introduces an example app for headless rendering, which however at the moment is not able to produce rendered images as originally intended.

There are two node.js libraries for headless WebGL rendering, headless-gl and node-gles.

headless-gl at the moment only supports WebGL 1.0.3 contexts, while glTF-Sample-Viewer requires at least WebGL 2.0. There is an open issue for WebGL 2.0 support on the headless-gl repository.

node-gles according to the developers is still under active development and not production ready. When using the node-gles package in the headless example app, node crashes with a segfault or produces an empty image. We assume this is due to the unfinished state of node-gles. Also some required image formats are not supported by node-gles yet (namely SRGB8_ALPHA8 and RGB32F).