Workaround video sync issue due to red5 timestamp offset

Red5 sometimes writes webcam video files with a large offset in the
video frame offsets, sometimes up to 30 or even 60 seconds. However,
the start event in the events.xml file corresponds to the time at
which red5 received the first keyframe (recorded frame) in the video.

The end result is that the video will sometimes appear to be
delayed (out of sync) in the processed recording.

The correction is simple: We're already reading video metadata,
including the timestamp of the first frame, so we just have to apply
a correction during video processing to undo the frame timestamp
offsets in the video file.