This performs the start code search which was previously part of
h264_find_frame_end() - the most CPU intensive part of the function.

By itself, this results in a performance regression:
              Before          After
              Mean   StdDev   Mean   StdDev  Change
Overall time  2925.6 26.2     3068.5 31.7    -4.7%

but this can more than be made up for by platform-optimised
implementations of the function.
Signed-off-by: Martin Storsjö <martin@martin.st>