Diagram of layering for encoding params

[robin-raymond]

$related #110

Think we could really benefit from a layering diagram at minimal for example 13, part III.

[aboba]

Here are a few layering diagrams:

1. 3-layer temporal scalability
2. 2-layer spatial simulcast with 2-layer temporal scalability
3. 2-layer spatial scalability with 2-layer temporal scalability

The difference between diagrams 2 ad 3 is subtle. See if you can figure out which is which :)