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 GObject macros either for GstFFMpegVidDec and GstFFMpegVidEnc can
break the compilation because they are not GTypes, since each av video
elements are registered in runtime.
https://bugzilla.gnome.org/show_bug.cgi?id=764162
As some libav encoders (such as MPEG2) use a thread_count parameter to control
how many threads to use, and since it was always being set to 0 (which uses
the default), suboptimal threading can sometimes be chosen. This extends the
libav encoders to allow for a max-threads parameter which is passed into
the internal structure to control this knob if applicable to the encoder.
https://bugzilla.gnome.org/show_bug.cgi?id=726612
