libavformat/utils.c has over 5500 lines and is supposed to contain
"various utility functions for use within FFmpeg". In reality it
contains all that and the whole demuxing+seeking core of libavformat.
This is especially bad, because said file includes the FFMPEG_VERSION
(the git commit sha) so that it is rebuilt whenever the commit HEAD
points to changes. Therefore this commit starts making it smaller
by factoring the seeking code out.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>