Add ability to export to ray tracing file formats

[ianmackenzie]

Possible renderers (could potentially support multiple ones!):

• Mitsuba
• YafaRay
• POV-Ray
• Appleseed
• pt
• Cycles (file format examples)
• Radiance
• MoonRay

Could either convert to a particular file format (like Mitsuba's XML format) and then render using a command-line tool, or encode as JSON and send to a server that decodes the JSON and then renders directly using a ray tracing library like pt.

[ianmackenzie]

Perhaps simplest/cleanest approach to start with: export to glTF, since it's very well documented, seems to map well onto elm-3d-scene concepts (simple built-in support for PBR materials etc.), and has pretty broad support.