Support glTF export in JS version

[tzanio]

⭐ It will be great to add support for the glTF export in glvis-4.2, which is bound to the key G. This is very similar to the PNG screenshots with key S, see e.g. VisualizationSceneSolution3d::glTF_Export().

🌟 Even cooler will be to add an “open in AR” button to the web version of GLVis that takes the current scene, makes the glTF file in memory, pipes it to modelviewer or similar and opens it in AR.

Here's an example