Add a -force_key_frames option to ffmpeg.

Add a -force_key_frames option to ffmpeg.

The option is useful to ensure that there is a seek point exactly at a
place the user will probably want to jump precisely sometime, the
major example would be the end of an opening and the beginning of a
chapter. The scene change detection system will often make it happen,
but not always for example if there is a fade-in.

See the thread:
Subject: [FFmpeg-devel] [PATCH] -force_key_frames option
Date: Tue, 12 Oct 2010 15:16:26 +0200

Patch by Nicolas George -mail nicolas,george,normalesup,org.

Originally committed as revision 25526 to svn://svn.ffmpeg.org/ffmpeg/trunk