Support custom Renderers

[DougHamil]

Discussed in https://github.com/DougHamil/threeagent/discussions/45

^{Originally posted by hkjels December 2, 2021} I believe three.js is possible to use with react-native, but I guess the render-method of threeagent will have to be changed right?

Allow a pre-created ThreeJS Renderer instance to be provided to the threeagent.core/render opts map. When a renderer is provided, no DOM canvas should be created.