Implement the force_fps option, which disables the automatic selection

of the best framerate amongst the list of supported framerates of the
codec. Use the av_find_nearest_q_idx() function to choose the best
framerate.

See the thread: "Implement in lavc a flag which makes avcodec_open()
to choose the best framerate".

Originally committed as revision 15445 to svn://svn.ffmpeg.org/ffmpeg/trunk