Is it possible to share contexts between renderers?

**What is the current behavior?** Hey 👋 I maintain [react-pdf](https://github.com/diegomura/react-pdf). Thanks for your awesome work and making `react-reconciler` for us to use! I've got many issues lately regarding context not working on my library and when doing tests I found out that context values aren't shared between renderers. This makes it impossible to share state such as themes, i18n, redux and more. As a bit of context, React-pdf is not a primary renderer, and as such, when used in the browser it runs on top of react-dom. I found the `isPrimaryRenderer` reconciler option that's supposed to be used for "multiple renderers concurrently render using the same context objects" but still any access of the context inside react-pdf components get's just the initial value (even if the context was updated with other value). The same happens for `react-art` that also set `isPrimaryRenderer=false`. **Minimal demo** I prepared a quick demo using react-art so you can see how it currently works: https://codesandbox.io/s/pedantic-hill-54kid?fontsize=14 **What is the expected behavior?** Share contexts between renderers when using `isPrimaryRenderer` config. Is there a way of achieving this? Am I missing something? **Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?** React: 16.11.0 React-dom: 16.11.0