Adds convenience definition for pictures that are field or mbaff based. Part of PAFF implementation.

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 10672 to svn://svn.ffmpeg.org/ffmpeg/trunk