Fix Picture.key_frame setting to be compatible with frame and field contexts. Part of PAFF implementation.

Contributed in part by Neil Brown.
patch by Jeff Downs, heydowns a borg d com
original thread:
Subject: [FFmpeg-devel] [PATCH] Implement PAFF in H.264
Date: 18/09/07 20:30

Originally committed as revision 10664 to svn://svn.ffmpeg.org/ffmpeg/trunk