Describe accurate threading model

[youennf]

Currently, we are not precisely describing the threading model and instead rely on pipeTo et al.
We should probably define a encoded media thread which is the thread on which happen the generation and consumption of frames.
And the thread of the window + the thread of the worker.
We could post/enqueue tasks between the various threads which would further clarify things.

[alvestrand]

The generation and consumption of frames is likely to happen on different threads than their JS processing, so any threading model that tries to include the generation and consumption itself is likely to be misleading.

We should concentrate on the events that concern frames appearing to the worker and what happens when the worker enqueues a frame (+ special events like thread end), and leave everything else as "out of scope for specification".

[alvestrand]

I see that #107 seems to describe the threading model the way I think it should be described (modulo my current lack of understanding of that PR).