Use multi-threading for rendering

[lassepe]

It would be nice to support multi-threaded rendering. Candidates for Threads.@spawn may be the render(t, dt) call in roll and (perhaps even more importantly) the call to writeframe. This might require some more restructuring since the push!(frames,...) design requires things to be rather sequential.

[lassepe]

I'd be happy to work on this and create a PR. However, first, I wanted to figure out whether there have been already some thought on this.

[MaximeBouton]

I have not thought about this but that would be great!
I thought the ffmpeg call was the bottleneck most of the time though.