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
The remaining use of CODEC_ are codec flags that has been moved into the
new codec private properties or have been deperated. Will be fixed in
later patches.
https://bugzilla.gnome.org/show_bug.cgi?id=792900
A AVCodecContext needs cleaning up before being freed.
Go through all of the allocations/setups to ensure none of them
can leak a context or its contents.
