Add externally driven rendering example

[atlv24]

Objective

• Show how to drive loop for rendering externally.

Solution

• Add example.

Testing

Produces:

[TimJentzsch]

Cool and useful example, though as someone who isn't deep into this topic, some more explanatory comments would be very helpful!

[tychedelia]

Agree re: more comments. Also, I think it might be helpful to call this externally_driven_headless_renderer? That might be too verbose, but there is some conceptual overlap with externally driven event looping (a la https://docs.rs/winit/latest/winit/platform/pump_events/trait.EventLoopExtPumpEvents.html). It's worth making clear that "external" here means no windows.