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

libavfilter/avfilter.h

@@ -105,6 +105,7 @@ typedef struct AVFilterPicRef
     int h;                      ///< image height
 
     int64_t pts;                ///< presentation timestamp in units of 1/AV_TIME_BASE
+    int64_t pos;                ///< byte position in stream, -1 if unknown
 
     AVRational pixel_aspect;    ///< pixel aspect ratio