ffdecoder: use avg_frame_rate instead of r_frame_rate

r_frame_rate is a guessed number defined as "the least common multiple
of all framerates in the stream". It has been deprecated and removed in
new Libav releases.

Use avg_frame_rate instead, which is the average framerate of the
stream.
add title on front-page
Merge branch 'master' of https://github.com/elenril/harvid