When opening files, switch back to avformat's own open function

Some not-well-behaved video files have the video "header" placed
at the end of the file.  When we open those files first in Julia,
and then pass the IO file descriptor to libavformat, it can't
find the header.  However, if we open the file directly with
libavformat, it is smart enough to seek to the end of the file
first and get the information.