avvidenc: Add thread-count parameter for libav encoders

avvidenc: Add thread-count parameter for libav encoders

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