gstreamer-libav doesn't build

Context

While looking into #234 (closed) I noticed that gstreamer-libav is not actually a dependency of anything in the SDK so hasn't actually built. Further it doesn't actually build against our version of ffmpeg.

Attempting to make things build I eventually got all the dependencies sorted and the configure to finish, but make failed with a whole host of compile errors. Notably there was an entirely missing header file libavfilter/avfiltergraph.h, which was removed from ffmpeg in April 2017, and a lot of changes to API, e.g. CODEC_FLAG_PASS2 is now AV_CODEC_FLAG_PASS2, and many others of a similar nature.

Acceptance Criteria

Gstreamer-libav builds successfully and runs correctly.