We were previously installing hardcoded properties for all
video encoders, refactor to instead use FFmpeg's AVOption API.

avvidenc still exposes a few properties related to the pass
mechanism: while the AVOption API allows specifying both passes
as flags at the same time, this is not practical in GStreamer's
context, where passes need to be run separately using a stats file.

https://bugzilla.gnome.org/show_bug.cgi?id=792900