vp8enc: extend support for temporal scalability, implement payloading support

Patches extracted from Pexip's gst-plugins-good branch.

For now, I have only extracted that set of patches, made it build on its own, ported to the future custom meta API (gstreamer!609 (merged)), made tests pass and modified a few things I spotted while doing so, but I still haven't given it an in-depth review.

From what I see the code looks sound and the mechanism to transport information from the encoder to the payloader makes sense to me, there are also quite a few unit tests, which is good. What I want to review in more detail is the payloading code wrt the RFC (https://tools.ietf.org/html/rfc7741#section-4.2), once I have done that I'll unWIP this.