async renders any message
Renderers can accpet a readyObs in ctx.
If they choose to support it, they set it to false until
they are done (meaning: won't cause flicker after this point)
RenderStack accepts a list of kvs or keys and renders all of them in parallel. An optional callback can be provided, ctx.readyObs semantics are also implemented, so RenderStacks can be used recursively.
License: AGBLv3 - Copyright 2019 Jan Boelsche