Improve fps detection in avformat producer.

Improve fps detection in avformat producer.

I noticed MLT sometimes gives wrong fps info (I can send some demo clips
if required), for example it gives a 1000.0 fps on some mp4 clips.

In december 2009, FFMpeg introduced avg_frame_rate that gives better
results than r_frame_rate which is currently used in producer_avformat.

Patch below makes use of this new field when available which gives
better results (my mpeg4 clip now shows a 22.691 fps instead of 1000.