instead of an ad-hoc function to search for start codes in order to
remove code duplication and to improve performance.

Improved performance of finding startcodes from 52606 decicycles to
9543 decicycles based upon 262144 runs for a 1 Mb/s MPEG4 video.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@googlemail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>