[Prototype] Intermediate Representation for Scene

[bryphe]

This is some prototyping for #411 to support a threaded renderer, and ability to support running part of Revery apps in a worker thread in JS.

Instead of Node's making direct calls against OpenGL in their draw methods - they'd return a set of instructions (an intermediate representation) - an immutable scene tree - that is serializable across native threads / JS workers.

This allows us to run take that immutable scene tree, and hand it off to a renderer thread to do the actual work of making the OpenGL calls (or in the future - Skia calls 😉 )

[bryphe]

Close for now - had to pursue an alternate strategy to get the playground working. This will be nice to have for a threaded draw experience though (can revisit once we have Skia in-place!)