Maintain pointer to end of AVFormatContext.packet_buffer list
This changes add_to_pktbuf() to maintain a pointer to the last entry
in the list, avoiding a linear walk-through on each call. Before this
change, add_to_pktbuf() could take a significant amount of time (10%
of total decoding time), even with input files of several minutes.
After the change, the time spent in this function is barely measurable
Originally committed as revision 14654 to svn://svn.ffmpeg.org/ffmpeg/trunk