Add AVFilterPicRef.pos field and make libavfilter propagate stream

Add AVFilterPicRef.pos field and make libavfilter propagate stream
byte position information, as stored in the pkt.pos, through the
filterchain.

Note that the pos field is added *non* at the end of the
AVFilterPicRef struct, thus breaking ABI compatibility, which is
allowed as the API is still considered non-stable.

Originally committed as revision 22506 to svn://svn.ffmpeg.org/ffmpeg/trunk